Add wheel support to sky-scrollable

sky/framework/sky-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.
 -->
 <import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement" />
 <import src="/sky/framework/fling-curve.sky" as="FlingCurve" />
 
 <sky-element
   name="sky-scrollable"
   on-gesturescrollstart="handleScrollStart_"
   on-gesturescrollend="handleScrollEnd_"
   on-gesturescrollupdate="handleScrollUpdate_"
   on-gestureflingstart="handleFlingStart_"
-  on-gestureflingcancel="handleFlingCancel_">
+  on-gestureflingcancel="handleFlingCancel_"
+  on-wheel="handleWheel_">
 <template>
   <style>
   :host {
     overflow: hidden;
     position: relative;
   }
   #scrollable {
     transform: translateY(0);
   }
   #vbar {
@@ skipping 82 matching lines @@
   }
 
   handleFlingStart_(event) {
     this.flingCurve_ = new FlingCurve(-event.velocityY, event.timeStamp);
     this.scheduleFlingUpdate_();
   }
 
   handleFlingCancel_(event) {
     this.stopFling_();
   }
+
+  handleWheel_(event) {
+    this.scrollBy(-event.offsetY);
+  }
 }.register();
 </script>
 </sky-element>