Remove virtual where not needed in TextTrackCue

Remove TextTrackCue::cueType() since it always returned CueType::WebVTT
and was never overriden. CueType::Generic was for TextTrackCueGeneric
which was removed in r158977.

Remove TextTrackCue::videoSizeDidChange(...) since it did nothing and
was never overridden. This looks like unfinished logic for moving cues
when the controls are shown and hidden, and could be revived in
<http://crbug.com/314066>.

BUG=313287

Review URL: https://codereview.chromium.org/50903009

git-svn-id: svn://svn.chromium.org/blink/trunk@161172 bbb929c8-8fbe-4397-9dbb-9b2b20218538