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This change adds a composited version of the bottom toolbar. The layer
is not specific to the bottom sheet and can be used for any bottom UI.
The layer is not added to the hierarchy in this patch.

BUG=815324

Change-Id: I309f392c975ed7099aaf7562461625bd7c886a2d
Reviewed-on: https://chromium-review.googlesource.com/935742
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543153}