Hide Picture-in-Picture button if video src is a MediaStream.

This CL makes sure native video controls don't show a Picture-in-Picture
button when video src is a MediaStream as backend is not implemented
yet.

Bug: 806249
Change-Id: I04171b2ba73e3689334809c6419bcc8ff598196f
Reviewed-on: https://chromium-review.googlesource.com/1145076
Reviewed-by: apacible <apacible@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#577083}
3 files changed