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This makes it clearer (everywhere) that the animation is simply animating the
alpha value of a single alert color.

Bug: none
Change-Id: Iad30d326dd3af811cc0e065d10ef637d32e5a94b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1914963
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715100}