Fix flattening for scrollable areas.

third_party/WebKit/LayoutTests/compositing/overflow/scroll-parent-with-non-stacking-context-composited-ancestor-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [98, 90],
           "bounds": [300, 300],
           "contentsOpaque": true,
           "drawsContent": true,
           "backfaceVisibility": "hidden",
           "backgroundColor": "#FFEFD5",
           "children": [
             {
               "bounds": [102, 102],
+              "shouldFlattenTransform": false,
               "drawsContent": true,
               "children": [
                 {
                   "position": [1, 1],
                   "bounds": [100, 100],
+                  "shouldFlattenTransform": false,
                   "children": [
                     {
-                      "bounds": [100, 180],
-                      "shouldFlattenTransform": false
+                      "bounds": [100, 180]
                     }
                   ]
                 }
               ]
             }
           ]
         },
         {
           "position": [60, 60],
           "bounds": [80, 80],
@@ skipping 26 matching lines @@
               "position": [94, 1],
               "bounds": [7, 100]
             }
           ]
         }
       ]
     }
   ]
 }