Stop using toParentMediaElement internally in MediaControlElements

Just use mediaElement()

BUG=341813
R=adamk@chromium.org

Review URL: https://codereview.chromium.org/196143005

git-svn-id: svn://svn.chromium.org/blink/trunk@169107 bbb929c8-8fbe-4397-9dbb-9b2b20218538