Add auto-scroll option to sky city-list example

sky/examples/city-list/city-list.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="../../framework/sky-element/sky-element.sky" as="SkyElement" />
 <import src="city-data-service.sky" as="CityDataService" />
 <import src="city-sequence.sky" as="CitySequence" />
 
 <template>
@@ skipping 581 matching lines @@
         var scrollFrame = this.scroller.getCurrentFrame();
         this.tiler.updateFirstItem(indexOffset);
         this.updateView(data.items, false);
       },
 
       handleScroll: function(event) {
         if (!this.scroller.captureNewFrame(event))
           return;
 
         this.updateView(this.loader.getItems(), true);
+      },
+
+      scrollBy: function(amount) {
+        this.scrollerElement.scrollTop += amount;
+        this.handleScroll({ target: this.scrollerElement });
       }
     });
 
 })(this);
 
 </script>