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Add new full screen capture function in accelerator, hide behind capture mode flag for now.

Test: Manual test
Bug: 1149725
Change-Id: I0ca1beb067aaa98d4c0099f6803515142ec5c616
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2572686
Commit-Queue: Judy Wang <shidi@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838726}