Fix subtle bugs in 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-element.sky" />
 
 <sky-element>
 <template>
   <style>
   :host {
     overflow: hidden;
     position: relative;
+    will-change: transform;
   }
   #scrollable {
     will-change: transform;
   }
   #vbar {
     position: absolute;
     right: 0;
     width: 3px;
     background-color: lightgray;
     pointer-events: none;
@@ skipping 103 matching lines @@
   }
 
   void _handleWheel(WheelEvent event) {
     scrollBy(-event.offsetY);
   }
 }
 
 _init(script) => register(script, SkyScrollable);
 </script>
 </sky-element>