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Add markdown doc for browser controls

Change-Id: I84c31c97a340011c42642810425c3e9676e27eef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1995877Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Sinan Sahin <sinansahin@google.com>
Cr-Commit-Position: refs/heads/master@{#804338}