[Picture-in-Picture] Disable feature if requirements are not fulfilled.

This CL makes sure document.pictureInPictureEnabled always returns false
and video.requestPictureInPicture() rejects if PictureInPicture and
UseSurfaceLayerForVideo features are not enabled.

Bug: 806249
Change-Id: Icc91903aa0f44dde577ce479a8d5d453952e6575
Reviewed-on: https://chromium-review.googlesource.com/937301
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540807}
14 files changed