Leaving Babylon playground triggers worker lifetime assertion

[jdm]

assertion failed: `(left == right)`
  left: `4`,
 right: `1`: This runtime still has live children. (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(1) }, at /Users/joshmatthews/.cargo/git/checkouts/rust-mozjs-8611526964119dd6/11cabde/src/rust.rs:424)
   0: backtrace::backtrace::trace_unsynchronized
   1: <servo::backtrace::Print as core::fmt::Debug>::fmt
   2: core::fmt::write
   3: std::io::Write::write_fmt
   4: servo::backtrace::print
   5: servo::main::{{closure}}
   6: std::panicking::rust_panic_with_hook
   7: rust_begin_unwind
   8: std::panicking::begin_panic_fmt
   9: <mozjs::rust::Runtime as core::ops::drop::Drop>::drop
  10: core::ptr::drop_in_place
  11: core::ptr::drop_in_place
  12: core::ptr::drop_in_place
  13: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
  14: core::ptr::drop_in_place
  15: core::ptr::drop_in_place
  16: <script::script_thread::ScriptThread as script_traits::ScriptThreadFactory>::create::{{closure}}
  17: std::sys_common::backtrace::__rust_begin_short_backtrace
  18: std::thread::Builder::spawn_unchecked::{{closure}}::{{closure}}
  19: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  20: std::panicking::try::do_call
  21: __rust_try
  22: std::panicking::try
  23: std::panic::catch_unwind
  24: std::thread::Builder::spawn_unchecked::{{closure}}
  25: core::ops::function::FnOnce::call_once{{vtable.shim}}
  26: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  27: std::sys::unix::thread::Thread::new::thread_start
  28: _pthread_start
[2020-05-05T19:02:03Z ERROR servo] assertion failed: `(left == right)`
      left: `4`,
     right: `1`: This runtime still has live children.

[gterzian]

Pretty sure this would have been fixed along with #25212