Initial scroll-blocks-on compositor integration

LayoutTests/compositing/layer-creation/scroll-blocks-on-script-expected.txt

+Verify that setting scroll-blocks-on from script immediately creates the necessary layer.
+
 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
+      "scrollBlocksOn": ["StartTouch", "WheelEvent"],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
-          "bounds": [100, 100],
-          "opacity": 0.5,
-          "contentsOpaque": true,
-          "drawsContent": true,
-          "backgroundColor": "#008000"
+          "bounds": [20, 20],
+          "scrollBlocksOn": ["ScrollEvent"]
         }
       ]
     }
   ]
 }