Paint local attachment backgrounds into composited scrolling contents layer

third_party/WebKit/LayoutTests/compositing/overflow/overflow-scroll-with-local-background-expected.html

 <script>
 onload = function() {
-    document.getElementById('scroller').scrollTo(0, 200);
+    document.getElementById('scroller').scrollTop = 200;
 }
 </script>
 <style>
 #scroller {
     background: gray local content-box;
     border: 10px solid rgba(0, 255, 0, 0.5);
     overflow: scroll;
     padding: 10px;
     width: 200px;
     height: 200px;
 }
 
 .spacer {
     height: 300px;
 }
 </style>
 <!-- #scroller has a locally attached background with padding so when it is
     scrolled we can see the background slide up. Doing this on a composited
     element without repainting requires the background to be painted into
     the scrolling contents layer and correctly account for that layer not
     including the border in its bounds. -->
 <div id="scroller">
     <div class="spacer"></div>
 </div>