Move VTT related files into vtt subdirectory, renaming for consistency.
Fix check style warnings on affected files.

BUG=319744

Review URL: https://codereview.chromium.org/68203028

git-svn-id: svn://svn.chromium.org/blink/trunk@162162 bbb929c8-8fbe-4397-9dbb-9b2b20218538