Split TextTrackCue exceptions test into TextTrackCue+VTTCue

This does not change anything, it is for purity reasons only.

BUG=none

Review URL: https://codereview.chromium.org/140573002

git-svn-id: svn://svn.chromium.org/blink/trunk@165235 bbb929c8-8fbe-4397-9dbb-9b2b20218538