Add suppressions for Pixel_WebGLSadCanvas

Bug: 835867, 575305
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: I03bd2083fc5df341ce0d4717d0c5af3abe822763
Tbr: kbr@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1028224Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553700}

content/test/gpu/gpu_tests/pixel_expectations.py

@@ -99,3 +99,7 @@ class PixelExpectations(GpuTestExpectations):
               ['linux', 'mac', 'win'], bug=744658)
     self.Fail('Pixel_CSS3DBlueBox_NoGpuProcess',
               ['linux', 'mac', 'win'], bug=744658)
+
+    # TODO(kbr): temporary suppression for new test.
+    self.Flaky('Pixel_WebGLSadCanvas', ['mac'], bug=575305)
+    self.Fail('Pixel_WebGLSadCanvas', ['android', 'nvidia'], bug=575305)