Intermittent crash in /_mozilla/css/iframe/size_attributes.html, /_mozilla/css/iframe/size_attributes_vertical_writing_mode.html and /html/semantics/embedded-content/the-iframe-element/iframe-with-base.html

[jdm]

  ▶ CRASH [expected PASS] /_mozilla/css/iframe/size_attributes.html
  │ 
  │ VMware, Inc.
  │ Gallium 0.4 on softpipe
  │ 3.3 (Core Profile) Mesa 12.0.1
  │ Shutting down the Constellation after generating an output file or exit flag specified
  │ called `Result::unwrap()` on an `Err` value: Error { repr: Custom(Custom { kind: NotFound, error: StringError("Bogus destination port.") }) } (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(1) }, at ../src/libcore/result.rs:837)
  │ called `Result::unwrap()` on an `Err` value: Error { repr: Custom(Custom { kind: NotFound, error: StringError("Bogus destination port.") }) } (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(1) }, at ../src/libcore/result.rs:837)
  │ called `Result::unwrap()` on an `Err` value: Error { repr: Custom(Custom { kind: NotFound, error: StringError("Bogus destination port.") }) } (thread LayoutThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(1) }, at ../src/libcore/result.rs:837)
  │ called `Result::unwrap()` on an `Err` value: Error { repr: Custom(Custom { kind: NotFound, error: StringError("Bogus destination port.") }) } (thread LayoutThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(1) }, at ../src/libcore/result.rs:837)
  │ VMware, Inc.
  │ Gallium 0.4 on softpipe
  │ 3.3 (Core Profile) Mesa 12.0.1
  │ Shutting down the Constellation after generating an output file or exit flag specified
  │ called `Result::unwrap()` on an `Err` value: Error { repr: Custom(Custom { kind: NotFound, error: StringError("Bogus destination port.") }) } (thread LayoutThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(1) }, at ../src/libcore/result.rs:837)
  │ stack backtrace:
  │    0:        0x1099bbfae - backtrace::backtrace::trace::h16372ee7bf1517e5
  │    1:        0x1099bc29c - backtrace::capture::Backtrace::new::hcc43c50c4b11c693
  │    2:        0x109686c64 - servo::main::_{{closure}}::h356eefa3f34c1614
  │    3:        0x10b066eb3 - std::panicking::rust_panic_with_hook::h00b81bb3dcbd51f2
  │    4:        0x10b066d84 - std::panicking::begin_panic::ha6a0d553db9869ff
  │    5:        0x10b066ca2 - std::panicking::begin_panic_fmt::h24d113aee3ee4081
  │    6:        0x10b066c07 - rust_begin_unwind
  │    7:        0x10b0939d0 - core::panicking::panic_fmt::he441b2ea2036b98a
  │    8:        0x10ac8c9b0 - core::result::unwrap_failed::h88381ff2d9927dee
  │    9:        0x10ac8d34d - profile_traits::mem::ProfilerChan::send::h859459806b43101d
  │   10:        0x1099e7734 - std::panicking::try::do_call::h43c066d67a7e9a69
  │   11:        0x10b067eda - __rust_maybe_catch_panic
  │   12:        0x109a0feb5 - _<F as alloc..boxed..FnBox<A>>::call_box::h97de36a403e35b2b
  │   13:        0x10b065d64 - std::sys::thread::Thread::new::thread_start::h9fa66f6812e81e10
  │   14:     0x7fff88879059 - _pthread_body
  │   15:     0x7fff88878fd6 - _pthread_start
  │ ERROR:servo: called `Result::unwrap()` on an `Err` value: Error { repr: Custom(Custom { kind: NotFound, error: StringError("Bogus destination port.") }) }
  │ Pipeline failed in hard-fail mode.  Crashing!
  │ thread panicked while processing panic. aborting.
  │ thread panicked while processing panic. aborting.
  └ thread panicked while processing panic. aborting.

[jdm]

This code has been rewritten to avoid calling unwrap on a fallible send operation.