Mark Pixel_OffscreenCanvasTransferToImageBitmap flaky on Windows.

Already marked flaky on Linux and Android.

NOTRY=true
TBR=xlai@chromium.org, trchen@chromium.org, ynovikov@chromium.org

Bug: 807742
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;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I883f6f0e21f20aebdff32071de29d4f02c477c72
Reviewed-on: https://chromium-review.googlesource.com/1006357
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549696}

content/test/gpu/gpu_tests/pixel_expectations.py

@@ -82,7 +82,7 @@ class PixelExpectations(GpuTestExpectations):
     self.Flaky('Pixel_OffscreenCanvasTransferAfterStyleResize',
               ['mac', 'linux', 'win', 'android'], bug=735171)
     self.Flaky('Pixel_OffscreenCanvasTransferToImageBitmap',
-              ['linux', 'android'], bug=807742)
+              ['linux', 'win', 'android'], bug=807742)
 
     self.Flaky('Pixel_OffscreenCanvasWebGLSoftwareCompositingWorker',
         ['mac', ('nvidia', 0xfe9), 'debug'], bug=751328)