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This is the first CL that implements the CaptureBarView. It only adds
the CaptureModeToggleButton, and the CaptureModeCloseButton, subsequent
CLs will build on these two views to implement the rest of the UI.

TBR=beccahughes@chromium.org
BUG=1112919

Change-Id: If9aca284bf419936df61ef0e285d1f143b06b0a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2336021
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795728}