This is the Blink-side code for a content privacy preference that blocks
HTML5 media elements <audio> and <video>. This is intended to work just
like the current setting for blocking images.

The LayoutTests in this CL rely on the Content Shell change in the CL
referenced below.

https://codereview.chromium.org/312103004/

BUG=50132

Review URL: https://codereview.chromium.org/27694002

git-svn-id: svn://svn.chromium.org/blink/trunk@177015 bbb929c8-8fbe-4397-9dbb-9b2b20218538