Update WebVTT parser spec references

Update the spec. references in the parser implementation to match the
ED from November 9 2013.

Only comments changed.

BUG=305317

Review URL: https://codereview.chromium.org/64273013

git-svn-id: svn://svn.chromium.org/blink/trunk@162127 bbb929c8-8fbe-4397-9dbb-9b2b20218538