Update SkiaRenderer Alpha Video Expectations

Some alpha blending media tests are flaky with SkiaRenderer. In both
Vulkan and GL.

This change adds a Skip expectation for GL. This change also updates
the note for the Vulkan expectations to refer to this being a bug and
not just lack of rebaselines.

TBR=backer@chromium.org
TEST=vulkan_native_blink_web_tests, skia_renderer_gl_blink_web_tests
media/alpha-video-playback.html
Bug: 1094635

Change-Id: Id8cab6764c88f7d21e889ca7dd8554e43de3554d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2324486Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792420}

third_party/blink/web_tests/FlagExpectations/use-gl=any

@@ -40,3 +40,6 @@ crbug.com/1009718 images/huge-image-viewport-scale.html [ Skip ]
 
 # Input or Playback flaky
 crbug.com/1060773 [ Win ] media/video-controls-visibility-multimodal-mouse-after-touch.html [ Skip ]
+
+# Alpha Blending Video Failures
+crbug.com/1094635 media/alpha-video-playback.html [ Skip ]

third_party/blink/web_tests/FlagExpectations/use-vulkan=native

@@ -20,6 +20,7 @@ crbug.com/993384 media/video-layer-crash.html [ Skip ]
 crbug.com/993384 transforms/transformed-document-element.html [ Skip ]
 crbug.com/993384 transforms/3d/point-mapping/3d-point-mapping-deep.html [ Skip ]
 
+# Alpha Blending Video Failures
 crbug.com/1094635 fast/canvas/canvas-pattern-video.html [ Skip ]
 crbug.com/1094635 media/alpha-video-playback.html [ Skip ]
 crbug.com/1094635 fast/canvas/canvas-composite-video-shadow.html [ Skip ]