Remove HTMLMediaElement.webkitHasClosedCaptions/webkitClosedCaptionsVisible

See the bug for why it is (probably) best to remove these.

Move some things in HTMLMediaElement.idl to match the spec order and
to leave all WebKit extensions in a group at the end.

Drop duplicate TreatNullAs=NullString introduced in r159260.

BUG=305753

Review URL: https://codereview.chromium.org/26759003

git-svn-id: svn://svn.chromium.org/blink/trunk@159317 bbb929c8-8fbe-4397-9dbb-9b2b20218538