Use TextResourceDecoder for the WebVTTParser

Simplifies BOM handling, signature detection and fixes handling
of "short" (< 9 bytes) inputs.

Also fixes the signature checker to allow a formfeed character to
follow the signature itself.

BUG=231460

Review URL: https://codereview.chromium.org/54203003

git-svn-id: svn://svn.chromium.org/blink/trunk@161049 bbb929c8-8fbe-4397-9dbb-9b2b20218538