Make generated Callback bindings classes FINAL

Make generated Callback bindings classes FINAL and add OVERRIDE to their virtual
methods.

R=haraken
BUG=333316

Review URL: https://codereview.chromium.org/138613002

git-svn-id: svn://svn.chromium.org/blink/trunk@165111 bbb929c8-8fbe-4397-9dbb-9b2b20218538