GitLab

This enables motion-stills by default and adds a feature flag for
turning it off.

Bug: 769140
Test: chrome --disable-features=ChangePictureVideoMode
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ia2eef2d627914572d95c91d08e876e5e37d282e6
Reviewed-on: https://chromium-review.googlesource.com/842703
Commit-Queue: David Reveman <reveman@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526052}