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This change:
1) Adds MenuClosureAnimationMac, which implements the Mac menu closure
   animation;
2) Adds logic to MenuItemView to force drawing the item in a selected
   or unselected state as needed, for use in this animation;
3) Adds support to MenuController for disabling handling input events,
   so that the closure animation can appear to be "synchronous";
4) Makes MenuController::Accept possibly asynchronous;
5) Updates unit tests to account for (4)

Bug: 829347
Change-Id: I8676718e6a5e9704b422ed4cb08e8d10002bf45a
Reviewed-on: https://chromium-review.googlesource.com/999803
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550160}