Change WebVTT to VTT in non-public API symbols.
This is a follow-up on https://codereview.chromium.org/68203028, which renamed files to use VTT.

BUG=320417

Review URL: https://codereview.chromium.org/65593003

git-svn-id: svn://svn.chromium.org/blink/trunk@162259 bbb929c8-8fbe-4397-9dbb-9b2b20218538