[Picture-in-Picture] Disable background video tracks optimization.

This CL makes sure a video player in Picture-in-Picture doesn't get
paused because of background video tracks optimization.

Bug: 806249, 663999
Change-Id: Ia3453000a9107d28974428c1ab004227370dc28f
Reviewed-on: https://chromium-review.googlesource.com/1008082
Reviewed-by: apacible <apacible@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#550534}
2 files changed