[Picture-in-Picture] Add Shadow DOM support for pictureInPictureElement

Until now, pictureInPictureElement was supported only in Document. This
CL makes it compliant with the spec by adding Shadow DOM support to
pictureInPictureElement. Note that I wish I could have used a partial
interface DocumentOrShadowRoot but partial interface for mix-in IDL are
not supported at that time (crbug.com/656517).

Bug: 806249, 656517
Change-Id: I5f9666ea9c52ded9a6d07a238fc7756371b7550e
Reviewed-on: https://chromium-review.googlesource.com/897513
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: apacible <apacible@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535747}
14 files changed