Fix subtle bugs in sky-scrollable

sky/framework/sky-drawer.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" />
 <import src="sky-scrollable.sky" />
 
 <sky-element>
 <template>
   <style>
     #mask {
       background-color: black;
       will-change: opacity;
       position: absolute;
       top: 0;
       left: 0;
       bottom: 0;
       right: 0;
     }
     #content {
-      background-color: yellow;
+      background-color: white;
       will-change: transform;
       position: absolute;
       width: 256px;
       top: 0;
       left: 0;
       bottom: 0;
     }
   </style>
   <div id="mask" />
   <div id="content">
@@ skipping 121 matching lines @@
       begin: currentPosition,
       end: targetPosition,
       duration: duration,
       curve: linear);
   }
 }
 
 _init(script) => register(script, SkyDrawer);
 </script>
 </sky-element>