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According to the latest UX mocks, Capture Mode shouldn't be started
if confidential content is currently visible on the screen.
So, it needs to be checked upfront while starting it.
However, it's still possible that a user will choose restricted area
for already initiated Capture Mode session, so the existing checks
should also be left.
See https://docs.google.com/presentation/d/1v0GM-lwHCznFJ6hnZlUH37eBOuAkJMw5Q6UnSkcKOgA/edit?hl=en#slide=id.g9ea2b2eb9f_9_256 for mocks.

Bug: 1133324
Change-Id: Iec4594b92118bf1e206bfafcadefd1e30fd070c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2501743
Commit-Queue: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Nikita Podguzov <nikitapodguzov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822115}