Tweak TextTrackCue/VTTCue virtual functions

Remove unreachable implementations and remove toString from release.

BUG=270340

Review URL: https://codereview.chromium.org/104473002

git-svn-id: svn://svn.chromium.org/blink/trunk@163184 bbb929c8-8fbe-4397-9dbb-9b2b20218538