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This CL draws/hides the focus rectangle around the PDF document based
on whether the PDF document has the focus or not.

The focus rectangle color is currently white. It's a placeholder for now
and will most likely change in future.

Fixed: 1069370
Change-Id: Ib8446a2326924ff0fed59d8fd2b04db462c1488c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2211562
Commit-Queue: Ankit Kumar 🌪️ <ankk@microsoft.com>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792439}