Implemented way to share video frames between WebKit and Chromium

Implemented way to share video frames between WebKit and Chromium

Implementing the chromium side of sharing video frames between WebKit
and Chromium. It is *not* intended to be committed as-is; it will need
to be broken up into several smaller pieces in order for both the WebKit
and the Chromium sides to be committed without breaking anything.

The WebKit stuff is added here for reading clarity and will be deleted from this patch when it is actually committed.

Please see https://bugs.webkit.org/show_bug.cgi?id=43590 for details on
patch.

BUG=48631
TEST=none