Add support for freezing iframes

Frames that aren't loading are not visible in the viewport will be frozen
if  --enable-features=FreezeFrames is provided. This will eventually be
converted to testing feature policies for frames.

BUG=907125

Change-Id: Ib0badb3f92df4ce4921dd4a4cb29c45749f64f47
Reviewed-on: https://chromium-review.googlesource.com/c/1450406
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629822}
21 files changed