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This CL implements the image capture part of this feature,
from all the possible sources, and handles saving the image
files and showing preview notifications as well as error
notifications.

Later CLs will implement logic to disable capture mode
operations such as policy or Data Leak Prevention (DLP).

BUG=1121699

Change-Id: I6dfba6f57adb0c48270b4d5ebb71e7d28fe489df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2376476
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804036}