Get rid of CSSStyleSheet special casing in the bindings generator

Get rid of CSSStyleSheet special casing in the bindings generator by
fixing the return type of CSSStyleSheet::ownerRule() method to match
the expected IDL type.

Review URL: https://chromiumcodereview.appspot.com/19007004

git-svn-id: svn://svn.chromium.org/blink/trunk@154103 bbb929c8-8fbe-4397-9dbb-9b2b20218538