Add a UseCounter for HTMLMediaElement.controller

MediaController isn't getting the attention it needs right now. Count
usage to help decide whether or not removal is feasible. Feasible does
not imply desirable, but the data can inform the decision.

Count a non-null controller without distinguishing how it was set
(markup or script) to measure the usage that affects behavior.

BUG=none

Review URL: https://codereview.chromium.org/156493002

git-svn-id: svn://svn.chromium.org/blink/trunk@166651 bbb929c8-8fbe-4397-9dbb-9b2b20218538