Fix flattening for scrollable areas.

third_party/WebKit/LayoutTests/compositing/overflow/clear-scroll-parent-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
           "bounds": [308, 208],
+          "shouldFlattenTransform": false,
           "drawsContent": true,
           "children": [
             {
               "position": [4, 4],
               "bounds": [285, 200],
+              "shouldFlattenTransform": false,
               "children": [
                 {
                   "bounds": [285, 530],
-                  "shouldFlattenTransform": false,
                   "drawsContent": true
                 }
               ]
             },
             {
               "bounds": [308, 208],
               "children": [
                 {
                   "position": [289, 4],
                   "bounds": [15, 185]
@@ skipping 45 matching lines @@
               "bounds": [100, 320],
               "drawsContent": true
             }
           ]
         }
       ]
     }
   ]
 }