WebVTT only allows <rt> if the currently innermost node is <ruby>

If <rt> is encountered and the current node isn't <ruby>, then drop the
<rt>.

Fixes tc023 to not expect an <rt>.

BUG=321038

Review URL: https://codereview.chromium.org/98783005

git-svn-id: svn://svn.chromium.org/blink/trunk@162985 bbb929c8-8fbe-4397-9dbb-9b2b20218538