Fix BiDi paragraph embedding level resolution for VTTCue

Iterate over code points rather than (UTF-16) code units.
Make determineTextDirection side-effect free and local to the compilation
unit.
Re-uses the BidiResolver to do the actual work, and removes the
concatenation of the textnodes.

BUG=322051

Review URL: https://codereview.chromium.org/81133002

git-svn-id: svn://svn.chromium.org/blink/trunk@162920 bbb929c8-8fbe-4397-9dbb-9b2b20218538