Fix Compositing of Opaque Scrolling Layers and Add Tests

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

 <script>
 if (window.testRunner)
-    window.testRunner.dumpAsText();
+    testRunner.dumpAsText();
+
+if (window.internals)
+    internals.runtimeFlags.compositeOpaqueScrollersEnabled = true;
 
 function hasOpaqueCompositedScrollingContentsLayer(layer) {
     if (layer.name == "Scrolling Contents Layer")
         return layer.contentsOpaque;
     if (layer.children) {
         for (var i = 0; i < layer.children.length; i++) {
             if (hasOpaqueCompositedScrollingContentsLayer(layer.children[i]))
                 return true;
         }
     }
     return false;
 }
 
 onload = function() {
     if (!window.testRunner || !window.internals)
         return;
 
     var result = "";
     result += "Should have opaque composited scrolling contents layer: ";
     if (hasOpaqueCompositedScrollingContentsLayer(JSON.parse(window.internals.layerTreeAsText(document))))
         result += "Pass.\n";
     else
         result += "Fail.\n";
-    window.testRunner.setCustomTextOutput(result);
+    testRunner.setCustomTextOutput(result);
 }
 </script>
 <style>
 #scroller {
     background: white local content-box;
     border: 10px solid rgba(0, 255, 0, 0.5);
     overflow: scroll;
     width: 200px;
     height: 200px;
-    will-change: transform;
 }
 .spacer {
     height: 300px;
 }
 </style>
 <!-- The scroller's Scrolling Contents Layer should be opaque due to the opaque
     color background. Even though the background has a content-box clip there
     is no gap because the scroller has no padding. This means that we will be
     able to draw text with subpixel anti-aliasing.
 -->
 <div id="scroller"><div class="spacer"></div></div>