GitLab

This forces autoplay on for all main frames, and turns on the autoplay
delegation mechanism via <iframe allow="autoplay" />. This turns off the
previous behavior of unconditionally allowing autoplay on all pages and
frames.

Bug: internal b/160651728
Test: Launch AMP Viewer via web sender, verify only the screen reader is
      active while videos/ads remain paused.

Change-Id: If570ca3911596bf27bcceb56360fad674a9236c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2296360Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Sean Topping <seantopping@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789315}