This test was added in https://chromium.googlesource.com/external/khronosgroup/webgl.git/+/02bd167a87074f6ae988878c4df94b90426f082e

https://build.chromium.org/p/chromium.gpu.fyi/builders/Android%20Release%20%28Nexus%206%29/builds/4352

WebglConformance_conformance_offscreencanvas_context_attribute_preserve_drawing_buffer (gpu_tests.webgl_conformance_integration_test.WebGLConformanceIntegrationTest) ... 
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: 0,0,0,0 should be 255,0,255,255. Was 0,0,0,0.
FAIL 0,0,0,0 should be 255,0,255,255. Was 0,0,0,0.