The error message is 
10/176] gpu_tests.webgl_conformance_integration_test.WebGLConformanceIntegrationTest.WebglConformance_conformance2_samplers_multi_context_sampler_test failed unexpectedly 6.0020s:
  
  Traceback (most recent call last):
    _RunGpuTest at content/test/gpu/gpu_tests/gpu_integration_test.py:73
      self.RunActualGpuTest(url, *args)
    RunActualGpuTest at content/test/gpu/gpu_tests/webgl_conformance_integration_test.py:203
      getattr(self, test_name)(test_path, *args[1:])
    _RunConformanceTest at content/test/gpu/gpu_tests/webgl_conformance_integration_test.py:217
      self._CheckTestCompletion()
    _CheckTestCompletion at content/test/gpu/gpu_tests/webgl_conformance_integration_test.py:213
      self.fail(self._WebGLTestMessages(self.tab))
    fail at /usr/lib/python2.7/unittest/case.py:412
      raise self.failureException(msg)
  AssertionError: should be green
  at (0, 0) expected: 0,255,0,255 was 0,0,0,255
  FAIL should be green
  at (0, 0) expected: 0,255,0,255 was 0,0,0,255
  
  Locals:
    msg : u'should be green\nat (0, 0) expected: 0,255,0,255 was 0,0,0,255\nFAIL should be green\nat (0, 0) expected: 0,255,0,255 was 0,0,0,255\n'


https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_optional_gpu_tests_rel/builds/8209

It came from this WebGL commit: https://github.com/KhronosGroup/WebGL/pull/2383
Assigning to test owner kbr@.

That test was added for  issue 713127 , I think this can be duped into it.