Paint local attachment backgrounds into composited scrolling contents layer

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

 <script>
 if (window.testRunner)
     testRunner.waitUntilDone();
 onload = function() {
     // Double rAF to ensure content is painted before scroll.
     requestAnimationFrame(function() {
         requestAnimationFrame(function() {
-            document.getElementById('scroller').scrollTo(0, 200);
+            document.getElementById('scroller').scrollTop = 200;
             if (window.testRunner)
                 window.testRunner.notifyDone();
         })
     });
 }
 </script>
 <style>
 #scroller {
-    background: gray local content-box;
+    background: url('../resources/apple.jpg') local;
     border: 10px solid rgba(0, 255, 0, 0.5);
     overflow: scroll;
     padding: 10px;
     width: 200px;
     height: 200px;
     will-change: transform;
 }
 
 .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. -->
+<!-- This scroller has a locally attached background image which should
+    scroll with the content. -->
 <div id="scroller">
     <div class="spacer"></div>
 </div>

[trchen, 2016/08/17 01:22:55]

Wow, this test actually failed without your CL. I tried toggling the background
image with devtools, it is definitely an under-invalidation bug. Could you
please file a bug and +cc wangxianzhu@ ? Thanks!