Disable flaky OffscreenCanvasTransferBeforeStyleResize test on windows.

This has been failing on Win 7 Debug (NVIDIA) and given the flakes on
other platforms it seems better to disable it on all Windows bots.

Failures seem to be for different reasons on alternate builds:
- DCHECK in OffscreenCanvasSurfaceReferenceFactory::SatisfySequence, or
- trying to draw invalid mailbox in browser compositor

Recent failures:
https://build.chromium.org/p/chromium.gpu/builders/Win7%20Debug%20%28NVIDIA%29/builds/52157
https://build.chromium.org/p/chromium.gpu/builders/Win7%20Debug%20%28NVIDIA%29/builds/52158
https://build.chromium.org/p/chromium.gpu/builders/Win7%20Debug%20%28NVIDIA%29/builds/52159
https://build.chromium.org/p/chromium.gpu/builders/Win7%20Debug%20%28NVIDIA%29/builds/52160

TBR=kbr
BUG=720188

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: I8538f7aa73a3d63feb2afdb0fda03c2a7af6233d
Reviewed-on: https://chromium-review.googlesource.com/511803
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#473801}

content/test/gpu/gpu_tests/pixel_expectations.py

@@ -69,6 +69,6 @@ class PixelExpectations(GpuTestExpectations):
               ['mac', ('intel', 0x0a2e)], bug=718183)
 
     self.Flaky('Pixel_OffscreenCanvasTransferBeforeStyleResize',
-              ['mac', 'linux'], bug=719186)
+              ['mac', 'linux', 'win'], bug=720188)
     self.Flaky('Pixel_OffscreenCanvasTransferAfterStyleResize',
               ['mac', 'win'], bug=719144)