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- Implement the view revealing animatee methods: when revealing the
thumb strip view, the toolbar's top right and top left corners will
become rounded.
- Add a pan gesture recognizer to reveal/hide the thumb strip by
dragging down/up the toolbar.

Bug: 1094335
Change-Id: Ib024cc033e2bf4f270c006330a3f972624b495eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315346
Commit-Queue: Roberto Moura <mouraroberto@google.com>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800059}