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Addition of the thumb strip coordinator, pan gesture
handler and animatee classes. The handler handles user pan gestures that
reveal/hide the thumb strip and calls methods on other UI elements to be
animated during thumb strip transitions. The view revealing animatee is
a protocol to be implemented by UI objects that are animated in these
transitions.

Bug: 1094335
Change-Id: Iad91af43ea4956b47f6784ea138fe4d81a482489
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2313877Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Roberto Moura <mouraroberto@google.com>
Auto-Submit: Roberto Moura <mouraroberto@google.com>
Cr-Commit-Position: refs/heads/master@{#799655}