Intermittent crash of /2dcontext/compositing/2d.composite.uncovered.nocontext.copy.html

[KiChjang]

  ▶ CRASH [expected OK] /2dcontext/compositing/2d.composite.uncovered.nocontext.copy.html
  │ 
  │ VMware, Inc.
  │ Gallium 0.4 on softpipe
  │ 3.3 (Core Profile) Mesa 12.0.1
  │ called `Result::unwrap()` on an `Err` value: IoError(Error { repr: Os { code: 104, message: "Connection reset by peer" } }) (thread main, at /checkout/src/libcore/result.rs:860)
  │ stack backtrace:
  │    0:     0x7f69c927607d - backtrace::backtrace::trace::h4e46a0cd30889b9c
  │    1:     0x7f69c9276562 - backtrace::capture::Backtrace::new::hb5a725a088a2a2fc
  │    2:     0x7f69c8c44b23 - servo::main::{{closure}}::h9d96b5c32a8afffa
  │    3:     0x7f69caa53a5d - std::panicking::rust_panic_with_hook
  │                         at /checkout/src/libstd/panicking.rs:546
  │    4:     0x7f69caa538a4 - std::panicking::begin_panic<collections::string::String>
  │                         at /checkout/src/libstd/panicking.rs:507
  │    5:     0x7f69caa53819 - std::panicking::begin_panic_fmt
  │                         at /checkout/src/libstd/panicking.rs:491
  │    6:     0x7f69caa537a7 - std::panicking::rust_begin_panic
  │                         at /checkout/src/libstd/panicking.rs:467
  │    7:     0x7f69caa8064d - core::panicking::panic_fmt
  │                         at /checkout/src/libcore/panicking.rs:69
  │    8:     0x7f69ca8ee608 - core::result::unwrap_failed::h6ff6bbf3069f9093
  │    9:     0x7f69ca8f0c30 - webrender_traits::api::RenderApi::get_scroll_layer_state::h9afa974f326da94e
  │   10:     0x7f69c8c32b25 - <compositing::compositor::IOCompositor<Window>>::send_viewport_rects::hefbb618dd9cb4af8
  │   11:     0x7f69c8c35424 - <compositing::compositor::IOCompositor<Window>>::handle_browser_message::h1cd07b146ce85a51
  │   12:     0x7f69c8c24637 - <servo::Browser<Window>>::handle_events::hfbc1c6982e8129ac
  │   13:     0x7f69c8c43945 - servo::main::hd292c4a1ab7c4d9b
  │   14:     0x7f69caa5aa2a - panic_unwind::__rust_maybe_catch_panic
  │                         at /checkout/src/libpanic_unwind/lib.rs:98
  │   15:     0x7f69caa541c6 - std::panicking::try<(),fn()>
  │                         at /checkout/src/libstd/panicking.rs:429
  │                          - std::panic::catch_unwind<fn(),()>
  │                         at /checkout/src/libstd/panic.rs:361
  │                          - std::rt::lang_start
  │                         at /checkout/src/libstd/rt.rs:57
  │   16:     0x7f69c6552f44 - __libc_start_main
  │   17:     0x7f69c8c1bea9 - <unknown>
  │   18:                0x0 - <unknown>
  │ ERROR:servo: called `Result::unwrap()` on an `Err` value: IoError(Error { repr: Os { code: 104, message: "Connection reset by peer" } })
  └ Pipeline failed in hard-fail mode.  Crashing!

[jdm]

This is #13479.