Mark Pixel_CSSFilterEffects failing on Mac/AMD 10.13.

This test became unacceptably flaky with the upgrade to 10.13.6.

Tbr: ynovikov@chromium.org
Bug: 872423
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: I8fded8d47792dce57ea6da6532f7d42b5c075bd4
Reviewed-on: https://chromium-review.googlesource.com/1172148Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582492}

content/test/gpu/gpu_tests/pixel_expectations.py

@@ -55,8 +55,9 @@ class PixelExpectations(GpuTestExpectations):
     self.Fail('Pixel_CSSFilterEffects',
         ['mac', ('nvidia', 0xfe9)], bug=690277)
 
-    # Became flaky on 10.13.6.
-    self.Flaky('Pixel_CSSFilterEffects', ['highsierra', 'amd'], bug=872423)
+    # Became flaky on 10.13.6. When it flakes, it flakes 3 times, so
+    # mark failing, unfortunately.
+    self.Fail('Pixel_CSSFilterEffects', ['highsierra', 'amd'], bug=872423)
 
     # TODO(kbr): flakily timing out on this configuration.
     self.Flaky('*', ['linux', 'intel', 'debug'], bug=648369)