Intermittent crash of /webvr/webvr-enabled-on-self-origin-by-feature-policy.https.sub.html

[KiChjang]

  ▶ CRASH [expected OK] /webvr/webvr-enabled-on-self-origin-by-feature-policy.https.sub.html
  │ 
  │ VMware, Inc.
  │ softpipe
  │ 3.3 (Core Profile) Mesa 18.3.0-devel
  │ Layout thread disconnected.: "SendError(..)" (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(1) }, at src/libcore/result.rs:997)
  │ 2019-02-27 02:13:02.971 servo[47483:659328251] Metadata.framework [Error]: couldn't get the client port
  │ stack backtrace:
  │    0:        0x106ffef9e - __ZN9backtrace9backtrace5trace17h9194fd11aa5e0269E
  │    1:        0x106ffdf2c - __ZN72_$LT$backtrace..capture..Backtrace$u20$as$u20$core..default..Default$GT$7default17h4bf15fdf689dde05E
  │    2:        0x104e0b369 - __ZN5servo4main28_$u7b$$u7b$closure$u7d$$u7d$17h932d539b161329d0E
  │    3:        0x107a6a170 - __ZN3std9panicking20rust_panic_with_hook17hdb6ee47c165b46dcE
  │    4:        0x107a69bbc - __ZN3std9panicking18continue_panic_fmt17h17fd7d2759b7149cE
  │    5:        0x107a69aa8 - _rust_begin_unwind
  │    6:        0x107a87771 - __ZN4core9panicking9panic_fmt17h254770d6c8ab828aE
  │    7:        0x105919142 - __ZN4core6result13unwrap_failed17h766eebc827e0e9ebE
  │    8:        0x10540ce78 - __ZN6script3dom6window6Window12force_reflow17h5d3eca058da6892aE
  │    9:        0x10540d51f - __ZN6script3dom6window6Window6reflow17h2159eb62bb5a95e8E
  │   10:        0x1056b0ced - __ZN6script3dom8document8Document11finish_load17h90a809e1d108927fE
  │   11:        0x1056d96fa - __ZN6script3dom11servoparser11ServoParser13do_parse_sync17ha0991831f1ef6f61E
  │   12:        0x1058ea41d - __ZN14profile_traits4time7profile17h0c5b2b47c4305455E
  │   13:        0x1056d8ffd - __ZN6script3dom11servoparser11ServoParser10parse_sync17h327817557b2b9a1cE.llvm.15429473194960033772
  │   14:        0x1056dd2b1 - __ZN93_$LT$script..dom..servoparser..ParserContext$u20$as$u20$net_traits..FetchResponseListener$GT$20process_response_eof17h29e1870afd670d6aE
  │   15:        0x1058b20b8 - __ZN6script13script_thread12ScriptThread29handle_msg_from_constellation17hbe9034c2f7b6825dE
  │   16:        0x1058aa6c5 - __ZN6script13script_thread12ScriptThread11handle_msgs17hc7872fa8fd1992edE.llvm.13491206658015953207
  │   17:        0x1059f2637 - __ZN14profile_traits3mem12ProfilerChan25run_with_memory_reporting17he1ccd80b8be7fa11E
  │   18:        0x1057f5e70 - __ZN3std10sys_common9backtrace28__rust_begin_short_backtrace17h277e380c8fb7669bE
  │   19:        0x1052bb92a - __ZN3std9panicking3try7do_call17h5c8095ab9bf5cebdE.llvm.13010053663604614974
  │   20:        0x107a733de - ___rust_maybe_catch_panic
  │   21:        0x10591b7e6 - __ZN50_$LT$F$u20$as$u20$alloc..boxed..FnBox$LT$A$GT$$GT$8call_box17h2210606d5d83f598E
  │   22:        0x107a7282b - __ZN3std3sys4unix6thread6Thread3new12thread_start17h2bafaa0015c96e6eE
  │   23:     0x7fff9838199c - __pthread_body
  │   24:     0x7fff98381919 - __pthread_start
  │ [2019-02-27T07:13:03Z ERROR servo] Layout thread disconnected.: "SendError(..)"
  └ Pipeline failed in hard-fail mode.  Crashing!

[jdm]

Adding this to the list in #22955.