Initial scroll-blocks-on compositor integration

LayoutTests/compositing/squashing/no-squashing-for-scroll-blocks-on-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
+      "scrollBlocksOn": ["StartTouch", "WheelEvent"],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
           "bounds": [100, 100],
           "contentsOpaque": true,
           "drawsContent": true,
           "backgroundColor": "#ADD8E6"
         },
         {
           "shouldFlattenTransform": false,
           "children": [
             {
-              "position": [-5, -5],
-              "bounds": [62, 62],
+              "bounds": [50, 50],
+              "scrollBlocksOn": ["ScrollEvent"],
+              "contentsOpaque": true,
               "drawsContent": true,
               "backgroundColor": "#D3D3D3"
             },
             {
               "position": [50, 50],
               "bounds": [50, 50],
               "drawsContent": true
             }
           ]
         }
       ]
     }
   ]
 }