[Picture-in-Picture] Disable/enable context menu when appropriate

The "Picture-in-Picture" video context menu is now enabled only if
Picture-in-Picture is enabled for this element.

Bug: 726621
Change-Id: If36b5881fdd16c910d399f2dab3ca36b7914c90e
Reviewed-on: https://chromium-review.googlesource.com/950945
Commit-Queue: François Beaufort <[email protected]>
Reviewed-by: apacible <[email protected]>
Reviewed-by: Istiaque Ahmed <[email protected]>
Reviewed-by: Mounir Lamouri <[email protected]>
Reviewed-by: Kentaro Hara <[email protected]>
Cr-Commit-Position: refs/heads/master@{#541407}
21 files changed