[Picture-in-Picture] Add PictureInPictureWindow resize event stub

This CL prepares the PictureInPictureWindow interface by adding the
|resize| event handler to PictureInPictureWindow. We currently don't
receive resize events from the backend. Triggering |resize| event will
come in a future CL.

Bug: 806249
Change-Id: Ib10eabc0e35af782dcdb36862535063d2dab0a2c
Reviewed-on: https://chromium-review.googlesource.com/908562
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: apacible <apacible@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536703}
6 files changed