Make MediaControls minimally virtual

Only the functions which are overridden in MediaControlsAndroid need
be virtual.

updateStatusDisplay() didn't do anything, so it was removed.

BUG=341813

Review URL: https://codereview.chromium.org/141323005

git-svn-id: svn://svn.chromium.org/blink/trunk@166907 bbb929c8-8fbe-4397-9dbb-9b2b20218538