Add wheel support to sky-scrollable

sky/examples/example-scrollable.sky

 <!--
 // Copyright 2015 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.
 -->
 <sky>
 <import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement" />
 <import src="/sky/framework/sky-scrollable.sky" />
 <import src="data/cities.sky" as="cities" />
 
@@ skipping 14 matching lines @@
     <template repeat="{{ cities }}">
       <template>
         <div>{{ name }}</div>
       </template>
     </template>
   </sky-scrollable>
 </template>
 <script>
 module.exports = class extends SkyElement {
   created() {
-    this.cities = cities;
+    this.cities = cities.slice(0, 300);
   }
 }.register();
 </script>
 </sky-element>
 
 <example-scrollable />
 </sky>