::cue should use ShadowPseudo relation.

video::cue() matches some element in a shadow tree hosted by <video>.
So CSSSelector should be ::cue -(ShadowPseudo)-> video.

This is almost the same behavior as custom pseudo element.

BUG=266226

TEST=no test, because this patch is preparation one for fixing apply-author-styles. By this patch, no behavior changes. When fixing apply-author-styles, will add a new test.

Review URL: https://chromiumcodereview.appspot.com/21331002

git-svn-id: svn://svn.chromium.org/blink/trunk@155252 bbb929c8-8fbe-4397-9dbb-9b2b20218538