Remove failure entries in gpu pixel test

Two of the newly added pixel tests have been picked up by all GPU bots.
One of the tests failed on Mac Retina with Nvidia GPU, it is likely due
to the device ratio, marking it as failure for now, and will fix it later.

BUG=563852
TBR=kbr@chromium.org
CQ_INCLUDE_TRYBOTS=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;master.tryserver.chromium.android:android_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2389313003
Cr-Commit-Position: refs/heads/master@{#423065}

content/test/gpu/gpu_tests/pixel_expectations.py

@@ -34,8 +34,8 @@ class PixelExpectations(GpuTestExpectations):
     # TODO(xlai) check / generate reference images.
     self.Fail('Pixel_OffscreenCanvasUnaccelerated2D', bug=563852)
     self.Fail('Pixel_OffscreenCanvasUnaccelerated2DWorker', bug=563858)
-    self.Fail('Pixel_OffscreenCanvasUnaccelerated2DGPUCompositing', bug=563852)
-    self.Fail('Pixel_OffscreenCanvasUnaccelerated2DGPUCompositingWorker', bug=563858)
+    self.Fail('Pixel_OffscreenCanvasUnaccelerated2DGPUCompositingWorker',
+              ['mac', ('nvidia', 0xfe9)], bug=652931)
 
     # TODO(kbr): flakily timing out on this configuration.
     self.Flaky('*', ['linux', 'intel', 'debug'], bug=648369)