Harden the compositor

[asajeffrey]

The constellation is hardened, but there are other single points of failure such as the compositor.

$ ./mach test-wpt --release --binary-arg=--random-pipeline-closure-probability=0.1 --binary-arg=--soft-fail --binary-arg=-M /DOMEvents
Running 25 tests in web-platform-tests

  ▶ CRASH [expected OK] /DOMEvents/init-event-while-dispatching.html
  │ 
  │ thread '<main>' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 32, message: "Broken pipe" } }', ../src/libcore/result.rs:746
  │ stack backtrace:
  │    1:     0x55686966b140 - sys::backtrace::tracing::imp::write::h495bc662e480d01f2cv
  │    2:     0x55686967008f - panicking::default_handler::_$u7b$$u7b$closure$u7d$$u7d$::closure.44522
  │    3:     0x55686966fd08 - panicking::default_handler::h4e73712d1e927dfeH0z
  │    4:     0x556869659aac - sys_common::unwind::begin_unwind_inner::h0e1be209f5e9e87dg2t
  │    5:     0x55686965aa58 - sys_common::unwind::begin_unwind_fmt::h45dc2fac9b5e0d23m1t
  │    6:     0x55686966a441 - rust_begin_unwind
  │    7:     0x5568696a7a8f - panicking::panic_fmt::h40f5ec0cdc3fc429FRL
  │    8:     0x556867a3d520 - result::unwrap_failed::h5029090533422012196
  │    9:     0x556867a547a9 - compositor::IOCompositor<Window>::send_viewport_rect_for_layer::h1376184889710214098
  │   10:     0x556867a499c4 - compositor::IOCompositor<Window>::handle_browser_message::h11410738671235407825
  │   11:     0x556867a44e74 - compositor::IOCompositor<Window>.CompositorEventListener::handle_events::h9443721467454960351
  │   12:     0x556867a7b36c - Browser::handle_events::hd718646c4d4147d1tea
  │   13:     0x556867a31275 - main::h9cc0aa5bb4f2f2aeqaa
  │   14:     0x55686966f964 - sys_common::unwind::try::try_fn::h11728613996190555864
  │   15:     0x55686966a3cb - __rust_try
  │   16:     0x55686966f3fb - rt::lang_start::h660c7a8519a9d7d5NSz
  │   17:     0x7feeeaee6a3f - __libc_start_main
  │   18:     0x556867a2f5c8 - _start
  └   19:                0x0 - <unknown>