implement autoplay policy to block media autoplay...

... when not allowed by content settings.

This commit adds three patches:

1. Implement experimental Autoplay site setting (behind a flag).
2. Add AllowAutoplay in WebContentSettingsClient (allows accessing the
   Autoplay setting in the renderer (blink).
3. Implement the below autoplay policy.

Current autoplay policy (if autoplay is blocked for site as per content
settings):
1. Muted autoplay is always blocked.
2. Autoplay with sound is allowed if and only if the user has interacted
   with domain (click, tap, etc.). If we block this, then the user won't
   be able to start a video by tapping on the play button, since those
   will be blocked too.

Author: octocorvus