Defer media playback in background tabs.

Videos which autoplay in the background will now have their load
deferred until the tab is visible for the first time -- this avoids
autoplay during session restore and premature playback.

Once a tab / RenderFrame has ever played media before, it's allowed
to continue to autoplay/autoload indefinitely; this is to support
playlist type applications.

TEST=manual verification that background tabs don't autoplay;
playlist type media continues to work.

Review URL:

Cr-Commit-Position: refs/heads/master@{#344092}
14 files changed