Adds src to the list of attributes known for iframes

sky/framework/sky-element/element-registry.sky

 <!--
 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 -->
 <import src="sky-binder.sky" as="binder" />
 <script>
 // TODO(esprehn): It would be nice if these were exposed by the platform so
 // the framework didn't need to hard code a list.
 var globalAttributesNames = new Set([
@@ skipping 147 matching lines @@
 
 registerElement('style', {
   'media': 'string',
 });
 
 registerElement('template', {
   'if': 'string',
   'repeat': 'string',
 });
 
+registerElement('iframe', {
+  'src': 'string',
+});
+
 module.exports = {
   registerElement: registerElement,
   getRegistration: getRegistration,
   checkAttribute: checkAttribute,
   isGlobalAttribute: isGlobalAttribute,
   isExpandableAttribute: isExpandableAttribute,
 };
 </script>