Prevent media controls from auto-hiding on audio-only content.

BUG=374959
TEST=LayoutTests/media/audio-controls-do-not-fade-out.html

Review URL: https://codereview.chromium.org/296743007

git-svn-id: svn://svn.chromium.org/blink/trunk@174587 bbb929c8-8fbe-4397-9dbb-9b2b20218538