Fix corner-cases for WebVTT tokenizer escape handling

Handle cases such as "&<", "&&", "&" followed by non-alphanumeric
characters (and similar variations).

Fix test tc022 which assumed that a single "&" would be ignored.

TEST=media/track/opera/track/webvtt/parsing-cue-data/tests/entities.html
BUG=321139

Review URL: https://codereview.chromium.org/97103002

git-svn-id: svn://svn.chromium.org/blink/trunk@162918 bbb929c8-8fbe-4397-9dbb-9b2b20218538