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Introduces a new root MVC component for bottom controls UI. This
component has a composited component to allow the bottom controls
to be scrolled offscreen. Currently only the split toolbar lives in
the bottom controls component but soon tab groups bottom toobar will
also live there.

Change-Id: I2070301da2faf6a2182dacf0a75b66b2bd97e61a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1496599
Commit-Queue: Pedro Amaral <amaralp@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638372}