Revert "Minimal fix to get cross frame tests working."

lib/dom/templates/html/frog/impl_IFrameElement.darttemplate

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
-$!MEMBERS
-
-  Window get _contentWindow() native "return this.contentWindow;";
-
-  // Override contentWindow to return secure wrapper.
-  Window get contentWindow() {
-    return _DOMWindowCrossFrameImpl._createSafe(_contentWindow);
-  }
-}