Let TextTrackResource be a RawResource

This allows deprecatedDidReceiveResource to be removed completely, by
using the exising appendData callback instead.

The removed FIXME was a little bit confused, in that it didn't seem to
realize that incremental parsing is needed for WebVTT.

Also add OVERRIDE where possible.

BUG=312756

Review URL: https://codereview.chromium.org/50123006

git-svn-id: svn://svn.chromium.org/blink/trunk@160848 bbb929c8-8fbe-4397-9dbb-9b2b20218538