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This CL achieve the animation by adding sequential labeled CSS class
to overflow menu list item so that we can add delays to each visible
items to let the items appear one after another.

Bug: 814507
Change-Id: I46ee3b6e6a8cbadaad33d2417454267ff4d62a66
Reviewed-on: https://chromium-review.googlesource.com/c/1357462
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613224}