Mark Pixel_OffscreenCanvasWebGLSoftwareCompositingWorker Flaky

on Mac Retina Debug (NVIDIA)

BUG=751328
TBR=kbr@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ic5f1b6ca6a83fdbf93b8bdda7fca86cc5c36176c
Reviewed-on: https://chromium-review.googlesource.com/597287
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491257}

content/test/gpu/gpu_tests/pixel_expectations.py

@@ -69,3 +69,7 @@ class PixelExpectations(GpuTestExpectations):
     self.Fail('Pixel_DirectComposition_Video_VP9', bug=615325)
     self.Fail('Pixel_Video_MP4', bug=615325)
     self.Fail('Pixel_Video_VP9', bug=615325)
+
+    self.Flaky('Pixel_OffscreenCanvasWebGLSoftwareCompositingWorker',
+        ['mac', ('nvidia', 0xfe9), 'debug'], bug=751328)
+