Something's fishy about that failure. The error log is:

[4/140] gpu_tests.webgl_conformance_integration_test.WebGLConformanceIntegrationTest.WebglConformance_conformance2_context_incorrect_context_object_behaviour failed unexpectedly 6.0345s:
  
  Traceback (most recent call last):
    _RunGpuTest at content/test/gpu/gpu_tests/gpu_integration_test.py:132
      self.RunActualGpuTest(url, *args)
    RunActualGpuTest at content/test/gpu/gpu_tests/webgl_conformance_integration_test.py:185
      getattr(self, test_name)(test_path, *args[1:])
    _RunConformanceTest at content/test/gpu/gpu_tests/webgl_conformance_integration_test.py:199
      self._CheckTestCompletion()
    _CheckTestCompletion at content/test/gpu/gpu_tests/webgl_conformance_integration_test.py:195
      self.fail(self._WebGLTestMessages(self.tab))
    fail at .swarming_module/lib/python2.7/unittest/case.py:410
      raise self.failureException(msg)
  AssertionError: contextA.isSync(syncB) should be false. Was true.
  FAIL contextA.isSync(syncB) should be false. Was true.
  
  Locals:
    msg : u'contextA.isSync(syncB) should be false. Was true.\nFAIL contextA.isSync(syncB) should be false. Was true.\n'


How often is this happening? I don't see a suppression for it and  Issue 858862  is preventing easy inspection of https://ci.chromium.org/p/chromium/builders/luci.chromium.try/linux_optional_gpu_tests_rel?limit=200 . Also, the flakiness dashboard hasn't picked up that failing run; right now this is empty:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webgl2_conformance_tests&tests=gpu_tests.webgl_conformance_integration_test.WebGLConformanceIntegrationTest.WebglConformance_conformance2_context_incorrect_context_object_behaviour

Oh, I see, this is failing when attempting to roll forward WebGL conformance in https://chromium-review.googlesource.com/1117894 .

Sorry, I don't know how that slipped by. The test was developed along with the fix.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a32faf137bbffbabf44ba29e50417d46c805ee19

commit a32faf137bbffbabf44ba29e50417d46c805ee19
Author: James Darpinian <jdarpinian@chromium.org>
Date: Thu Jun 28 23:46:23 2018

Roll WebGL 5e83981..a5c263c

https://chromium.googlesource.com/external/khronosgroup/webgl.git/+log/5e83981..a5c263c

Bug:  853978 ,  857303 
Change-Id: I9cf554d30f5f51bcdca25f7fa6b1027312de0e77
Cq-Include-Trybots: luci.chromium.try:win_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_angle_rel_ng;luci.chromium.try:win_angle_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/1117894
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571327}
[modify] https://crrev.com/a32faf137bbffbabf44ba29e50417d46c805ee19/DEPS
[modify] https://crrev.com/a32faf137bbffbabf44ba29e50417d46c805ee19/content/test/gpu/gpu_tests/webgl2_conformance_expectations.py
[modify] https://crrev.com/a32faf137bbffbabf44ba29e50417d46c805ee19/content/test/gpu/gpu_tests/webgl_conformance_revision.txt

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0957c69823ea5c29c9dbb902204759f140f34015

commit 0957c69823ea5c29c9dbb902204759f140f34015
Author: Kenneth Russell <kbr@chromium.org>
Date: Wed Aug 08 23:08:46 2018

Verify WebGLSync objects in isSync() API.

Missed during the last change to this code. Unsuppress the associated
WebGL conformance test.

Bug:  857303 
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Iefb2f90e6c440ae5eabc1bedd4171fbfe9a3dd0a
Reviewed-on: https://chromium-review.googlesource.com/1166454
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581719}
[modify] https://crrev.com/0957c69823ea5c29c9dbb902204759f140f34015/content/test/gpu/gpu_tests/webgl2_conformance_expectations.py
[modify] https://crrev.com/0957c69823ea5c29c9dbb902204759f140f34015/third_party/blink/renderer/modules/webgl/webgl2_rendering_context_base.cc