[Picture-in-Picture] Add basic implementation for document.pictureInPictureElement

This doesn't handle ShadowRoot yet. Full implementation will come in a future CL.

Bug: 806249
Change-Id: I948b8f355549d759790545248593a37acb567579
Reviewed-on: https://chromium-review.googlesource.com/890740
Commit-Queue: Mounir Lamouri (slow) <mlamouri@chromium.org>
Reviewed-by: Mounir Lamouri (slow) <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533639}
11 files changed