Skip dual-GPU switching tests on Mac NVIDIA.

This configuration's been forced to use only the integrated GPU due to
graphics driver bug workarounds, so these tests don't pass on this
configuration.

Tbr: enga@chromium.org
Bug: 1047573
No-Try: True
Change-Id: Id6e436ae118a1fbb552cfebd995f689b274f893e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037826
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738409}

content/test/gpu/gpu_tests/test_expectations/pixel_expectations.txt

@@ -218,8 +218,12 @@ crbug.com/1019462 [ android nvidia no-skia-renderer ] Pixel_Video_MP4_Rounded_Co
 crbug.com/1019462 [ android nvidia no-skia-renderer ] Pixel_Video_Context_Loss_VP9 [ RetryOnFailure ]
 crbug.com/1019462 [ android nvidia no-skia-renderer ] Pixel_WebGL2_BlitFramebuffer_Result_Displayed [ RetryOnFailure ]
 
-# Skip dual gpu switch tests on Mac Nvidia
-crbug.com/681341 [ mac nvidia ] Pixel_WebGLLowToHighPower [ Skip ]
+# Skip dual gpu switching tests on Mac Nvidia because this
+# configuration has been forced to use only the low-power GPU due to
+# graphics driver bugs.
+crbug.com/1047573 [ mac nvidia ] Pixel_WebGLLowToHighPower [ Skip ]
+crbug.com/1047573 [ mac nvidia ] Pixel_WebGLLowToHighPowerAlphaFalse [ Skip ]
+crbug.com/1047573 [ mac nvidia ] Pixel_WebGLHighToLowPower [ Skip ]
 
 # Flakes on Linux, Mac and Windows
 crbug.com/1028975 [ linux ] Pixel_PrecisionRoundedCorner [ RetryOnFailure ]