Clarify that there is no :cue or :cue(...) pseudo-class

The test passes with or without these changes, demonstrating that the
::cue pseudo-element is already the only one, as per the spec:
http://dev.w3.org/html5/webvtt/#the-cue-pseudo-element

TEST=LayoutTests/media/track/track-css-cue-pseudo-class.html

Review URL: https://codereview.chromium.org/677943002

git-svn-id: svn://svn.chromium.org/blink/trunk@184585 bbb929c8-8fbe-4397-9dbb-9b2b20218538