Support the 'lrm', 'rlm' and 'nbsp' entities in the WebVTTTokenizer

TEST=media/track/track-webvtt-tc022-entities.html
BUG=314028

Review URL: https://codereview.chromium.org/66503013

git-svn-id: svn://svn.chromium.org/blink/trunk@161822 bbb929c8-8fbe-4397-9dbb-9b2b20218538