Remove WebVTT checks from SharedStyleFinder

WebVTTElement does not return true from isStyledElement() so supportsStyleSharing
will always be false and it will never be a candidate. This means we don't need
to check anything related to WebVTT in SharedStyleFinder.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/34283003

git-svn-id: svn://svn.chromium.org/blink/trunk@160223 bbb929c8-8fbe-4397-9dbb-9b2b20218538