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This CL fills in more impl of the capture session:
- Paints the capture region according to the current selected
  source.
- Add support for event handling.
- Pressing ESC dismisses the session, while ENTER performs
  the capture and dismisses the session.

BUG=1116137
TEST=Manual

Change-Id: Ic86fd1230dea04d8d59ed19cbf7455e70d0a7795
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354567Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798276}