Update media/alpha-video-playback.html FlagExpectations

A crash has been introduced in media/alpha-video-playback.html
when running with SkiaRenderer and SwiftShader. This change updates
the flag expectations to skip the test. Until the root cause can be fixed.

TBR=backer@chromium.org
Test=skia_renderer_swiftshader_blink_web_tests
media/alpha-video-playback.html

Bug: 988120
Change-Id: I60d8eee0ab5c253ffe2900ac2effdfbebb090abb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730272Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683035}

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

@@ -3,3 +3,6 @@ crbug.com/972621 media/controls/paint-controls-webkit-appearance-none-custom-bg.
 crbug.com/972621 media/controls/paint-controls-webkit-appearance-none.html [ Skip ]
 crbug.com/972621 media/video-no-audio.html [ Skip ]
 crbug.com/972621 transforms/transformed-document-element.html [ Skip ]
+
+# Crash in BlinkTestController::OnPixelDumpCaptured
+crbug.com/988120 media/alpha-video-playback.html [ Skip ]
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