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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}