Intermittent crashes in tests that use webrender after it's shut down

[jdm]

1 unexpected results that are NOT known-intermittents:
  â–¶ CRASH [expected PASS] /_mozilla/mozilla/worklets/test_paint_worklet_timeout.html
  │ 
  │ error: XDG_RUNTIME_DIR not set in the environment.
  │ libEGL warning: No hardware driver found, falling back to software rendering
  │ Shutting down the Constellation after generating an output file or exit flag specified
  │ called `Result::unwrap()` on an `Err` value: RecvError (thread CanvasThread, at components/servo/lib.rs:957)
  │    0: servo::backtrace::print
  │    1: servo::main::{{closure}}
  │    2: std::panicking::rust_panic_with_hook
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libstd/panicking.rs:481
  │    3: rust_begin_unwind
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libstd/panicking.rs:385
  │    4: core::panicking::panic_fmt
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libcore/panicking.rs:89
  │    5: core::option::expect_none_failed
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libcore/option.rs:1272
  │    6: <servo::CanvasWebrenderApi as canvas::canvas_paint_thread::WebrenderApi>::generate_key
  │    7: canvas::canvas_data::CanvasData::send_data
  │    8: std::sys_common::backtrace::__rust_begin_short_backtrace
  │    9: core::ops::function::FnOnce::call_once{{vtable.shim}}
  │   10: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/liballoc/boxed.rs:1034
  │       <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/liballoc/boxed.rs:1034
  │       std::sys::unix::thread::Thread::new::thread_start
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libstd/sys/unix/thread.rs:87
  │   11: start_thread
  │   12: __clone
  │ [2020-06-10T13:46:13Z ERROR servo] called `Result::unwrap()` on an `Err` value: RecvError
  │ called `Option::unwrap()` on a `None` value (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(1) }, at components/script/script_thread.rs:1083)
  │    0: servo::backtrace::print
  │    1: servo::main::{{closure}}
  │    2: std::panicking::rust_panic_with_hook
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libstd/panicking.rs:481
  │ index out of bounds: the len is 0 but the index is 419 (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(1) }, at /root/.cargo/registry/src/github.com-1ecc6299db9ec823/addr2line-0.12.0/src/lib.rs:937)
  │    0: servo::backtrace::print
  │    1: servo::main::{{closure}}
  │    2: std::panicking::rust_panic_with_hook
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libstd/panicking.rs:481
  │    3: rust_begin_unwind
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libstd/panicking.rs:385
  │    4: core::panicking::panic_fmt
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libcore/panicking.rs:89
  │    5: core::panicking::panic_bounds_check
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libcore/panicking.rs:65
  │    6: addr2line::FrameIter<R>::next
  │    7: backtrace::symbolize::gimli::resolve
  │    8: <servo::backtrace::Print as core::fmt::Debug>::fmt::{{closure}}
  │    9: backtrace::backtrace::libunwind::trace::trace_fn
  │   10: _Unwind_Backtrace
  │   11: <servo::backtrace::Print as core::fmt::Debug>::fmt
  │   12: core::fmt::write
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libcore/fmt/mod.rs:1069
  │   13: std::io::Write::write_fmt
  │   14: servo::backtrace::print
  │   15: servo::main::{{closure}}
  │   16: std::panicking::rust_panic_with_hook
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libstd/panicking.rs:481
  │   17: rust_begin_unwind
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libstd/panicking.rs:385
  │   18: core::panicking::panic_fmt
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libcore/panicking.rs:89
  │   19: core::panicking::panic
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libcore/panicking.rs:52
  │   20: std::thread::local::LocalKey<T>::with
  │   21: <script::dom::worklet::Worklet as core::ops::drop::Drop>::drop
  │   22: script::dom::bindings::codegen::Bindings::WorkletBinding::WorkletBinding::_finalize::{{closure}}
  │   23: mozjs::panic::wrap_panic
  │   24: script::dom::bindings::codegen::Bindings::WorkletBinding::WorkletBinding::_finalize
  │   25: _ZNK7JSClass10doFinalizeEP8JSFreeOpP8JSObject
  │              at /repo/target/release/build/mozjs_sys-67fb07517d2b6e3e/out/build/dist/include/js/Class.h:831
  │       _ZN8JSObject8finalizeEP8JSFreeOp
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/vm/JSObject-inl.h:69
  │       _ZN2js2gc5Arena8finalizeI8JSObjectEEmP8JSFreeOpNS0_9AllocKindEm
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:472
  │   26: _ZL19FinalizeTypedArenasI8JSObjectEbP8JSFreeOpPPN2js2gc5ArenaERNS4_15SortedArenaListENS4_9AllocKindERNS3_11SliceBudgetE
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:528
  │   27: _ZN2js2gc10ArenaLists18foregroundFinalizeEP8JSFreeOpNS0_9AllocKindERNS_11SliceBudgetERNS0_15SortedArenaListE
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:5628
  │   28: _ZN2js2gc9GCRuntime17finalizeAllocKindEP8JSFreeOpRNS_11SliceBudgetE
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:5951
  │   29: _ZN11sweepaction18SweepActionForEachI13ContainerIterIN7mozilla7EnumSetIN2js2gc9AllocKindEmEEES7_E3runERNS5_11SweepAction4ArgsE
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:6177
  │   30: _ZN11sweepaction19SweepActionSequence3runERN2js2gc11SweepAction4ArgsE
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:6142
  │   31: _ZN11sweepaction18SweepActionForEachIN2js2gc19SweepGroupZonesIterEP9JSRuntimeE3runERNS2_11SweepAction4ArgsE
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:6177
  │   32: _ZN11sweepaction19SweepActionSequence3runERN2js2gc11SweepAction4ArgsE
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:6142
  │   33: _ZN11sweepaction18SweepActionForEachIN2js2gc15SweepGroupsIterEP9JSRuntimeE3runERNS2_11SweepAction4ArgsE
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:6177
  │   34: _ZN2js2gc9GCRuntime19performSweepActionsERNS_11SliceBudgetE
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:6310
  │   35: _ZN2js2gc9GCRuntime16incrementalSliceERNS_11SliceBudgetERKN7mozilla5MaybeI18JSGCInvocationKindEEN2JS8GCReasonERNS0_13AutoGCSessionE
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:6830
  │   36: _ZN2js2gc9GCRuntime7gcCycleEbNS_11SliceBudgetERKN7mozilla5MaybeI18JSGCInvocationKindEEN2JS8GCReasonE
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:7243
  │   37: _ZN2js2gc9GCRuntime7collectEbNS_11SliceBudgetERKN7mozilla5MaybeI18JSGCInvocationKindEEN2JS8GCReasonE
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:7428
  │   38: _ZN2js2gc9GCRuntime2gcE18JSGCInvocationKindN2JS8GCReasonE
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/gc/GC.cpp:7510
  │   39: _ZN9JSRuntime14destroyRuntimeEv
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/vm/Runtime.cpp:285
  │   40: _ZN2js14DestroyContextEP9JSContext
  │              at /root/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/vm/JSContext.cpp:198
  │   41: <mozjs::rust::Runtime as core::ops::drop::Drop>::drop
  │   42: core::ptr::drop_in_place
  │   43: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
  │   44: core::ptr::drop_in_place
  │   45: std::sys_common::backtrace::__rust_begin_short_backtrace
  │   46: core::ops::function::FnOnce::call_once{{vtable.shim}}
  │   47: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/liballoc/boxed.rs:1034
  │       <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/liballoc/boxed.rs:1034
  │       std::sys::unix::thread::Thread::new::thread_start
  │              at /rustc/a74d1862d4d87a56244958416fd05976c58ca1a8/src/libstd/sys/unix/thread.rs:87
  │   48: start_thread
  │   49: __clone
  │ [2020-06-10T13:46:13Z ERROR servo] index out of bounds: the len is 0 but the index is 419
  │ thread panicked while panicking. aborting.
  â”” Stack trace for thread "ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(1) }"