REGRESSION(r153039): Media controls should not have focus.

r153039 made buttons mouse-focusable, but audio/video elements should not have
focus at all.

BUG=262896

Review URL: https://chromiumcodereview.appspot.com/20029003

git-svn-id: svn://svn.chromium.org/blink/trunk@154814 bbb929c8-8fbe-4397-9dbb-9b2b20218538