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It uses a flood fill ripple that remains active while the overflow shelf
is active. InkDropHostView and CustomButton are updated to handle active
case:
 - CustomButton does not hide the ripple if it is not a pending ripple;
 - InkDropHostView does not show or hide a pending ripple if it is
   showing an active ripple.

BUG=612579
TEST=OverflowButtonInkDropTest.* in ash_unittests, manual

Review-Url: https://codereview.chromium.org/2178163002
Cr-Commit-Position: refs/heads/master@{#419232}