Add "intrinsicSize" attribute to HTMLVideoElement behind a flag

This CL is based on: https://chromium-review.googlesource.com/c/chromium/src/+/1150725

Design doc: https://docs.google.com/document/d/1yh_-ayeaFV0EjuR51U641zbrPAB0Baqj6UrC9bT9iqQ/edit#heading=h.y7amr11fn0tb

This CL implements "intrinsicSize" attribute for HTMLVideoElement.
The value of "intrinsicSize" overrides the intrinsic size:

1. When no width and (or) height is specified, the value of
"intrinsicSize" will be used to determine the area to layout the video
content.

2. videoWidth and videoHeight will return the value defined by
"intrinsicSize".

See explainer: https://github.com/ojanvafai/intrinsicsize-attribute/blob/master/README.md

Bug: 874629
Change-Id: If4aeceee2299c50aa7b4c9bdf8fef7cb3e7eea70
Reviewed-on: https://chromium-review.googlesource.com/1191926
Commit-Queue: Luna Lu <loonybear@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587667}
9 files changed