Remove useless ActiveDOMObject overrides

These overrides don't do anything except forward to the base class.  We can
remove the OVERRIDE.

Review URL: https://chromiumcodereview.appspot.com/24909003

git-svn-id: svn://svn.chromium.org/blink/trunk@158427 bbb929c8-8fbe-4397-9dbb-9b2b20218538