Script thread dies calling has_css_layout_box

[jdm]

Open https://sketchfab.com/models/311d052a9f034ba8bce55a1a8296b6f9 in master.

godot:master-servo jdm$ ./mach run -r https://sketchfab.com/models/311d052a9f034ba8bce55a1a8296b6f9
ERROR 2018-05-16T17:33:49Z: script::dom::bindings::error: Error at https://static.sketchfab.com/static/builds/web/dist/viewer-e5b1634d62fa06805afd.js:12:141827 navigator.vendor is undefined
ERROR 2018-05-16T17:33:54Z: offscreen_gl_context::draw_buffer: The given GLContext doesn't support requested antialising
called `Result::unwrap()` on an `Err` value: "SendError(..)" (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(1) }, at libcore/result.rs:945)
stack backtrace:
   0:        0x10591d33e - backtrace::backtrace::trace::h7964d766053cc9d7
   1:        0x10591cb2c - backtrace::capture::Backtrace::new::h4b3e83f668a53406
   2:        0x1025f0a2e - servo::main::_$u7b$$u7b$closure$u7d$$u7d$::h4da6d5caa0320835
   3:        0x105947194 - std::panicking::rust_panic_with_hook::hcebe05e8466ff134
   4:        0x105946c80 - std::panicking::begin_panic_fmt::hf6867449b867ca07
   5:        0x105946c42 - rust_begin_unwind
   6:        0x105958f45 - core::panicking::panic_fmt::h794f4a50949acedc
   7:        0x1034d480d - core::result::unwrap_failed::hc496027f07ad3429
   8:        0x103830141 - script::dom::window::Window::force_reflow::haec3ce39bc921c3e
   9:        0x103830782 - script::dom::window::Window::reflow::h1b0d5f4dee259279
  10:        0x1035e089c - script::dom::element::Element::has_css_layout_box::h9ce3e430edd4a451
  11:        0x102e6d0d1 - script::script_thread::ScriptThread::handle_msg_from_constellation::h8bd25f8571298553
  12:        0x102e689c5 - script::script_thread::ScriptThread::handle_msgs::h3f1444a335136b97 (.llvm.14170453006689695667)
  13:        0x10329a6c7 - profile_traits::mem::ProfilerChan::run_with_memory_reporting::hb3ae35d5ffcf2275
  14:        0x1035bbc8a - std::sys_common::backtrace::__rust_begin_short_backtrace::h91551003c0b69716
  15:        0x102aa26bd - std::panicking::try::do_call::h7320186ba2ee39e6 (.llvm.12154743355123556234)
  16:        0x10595297e - __rust_maybe_catch_panic
  17:        0x10380de36 - _$LT$F$u20$as$u20$alloc..boxed..FnBox$LT$A$GT$$GT$::call_box::h3a845036738de739
  18:        0x1059426f7 - std::sys_common::thread::start_thread::h3a2a1586806f3926
  19:        0x10592b628 - std::sys::unix::thread::Thread::new::thread_start::h657bd8405a5d884c
  20:     0x7fffbfb9093a - _pthread_body
  21:     0x7fffbfb90886 - _pthread_start
ERROR 2018-05-16T17:33:55Z: servo: called `Result::unwrap()` on an `Err` value: "SendError(..)"