Mark Pixel_WebGL_PremultipliedAlpha_False failing on NVIDIA SHIELD TV.

Needed to get this bot green again.

BUG=791733
TBR=ynovikov@chromium.org
NOTRY=true

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: If14d4b875f650c2d7c4fb82393d4b3c3aad509cb
Reviewed-on: https://chromium-review.googlesource.com/808124Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521650}

content/test/gpu/gpu_tests/pixel_expectations.py

@@ -92,3 +92,7 @@ class PixelExpectations(GpuTestExpectations):
         ['highsierra', ('intel', 0xa2e)], bug=774809)
     self.Fail('Pixel_WebGLGreenTriangle_NonChromiumImage_NoAA_NoAlpha',
         ['highsierra', ('intel', 0xa2e)], bug=774809)
+
+    # Failing on NVIDIA Shield TV; not sure why yet.
+    self.Fail('Pixel_WebGL_PremultipliedAlpha_False',
+              ['android', 'nvidia'], bug=791733)