Fix flattening for scrollable areas.

third_party/WebKit/LayoutTests/compositing/squashing/composited-bounds-for-negative-z-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "children": [
         {
           "bounds": [800, 408],
           "drawsContent": true,
           "children": [
             {
               "position": [8, 100],
               "bounds": [10, 10],
               "contentsOpaque": true,
               "drawsContent": true,
               "backgroundColor": "#FF0000"
             },
             {
               "bounds": [800, 408],
               "drawsContent": true
             },
             {
               "position": [108, 100],
               "bounds": [300, 300],
+              "shouldFlattenTransform": false,
               "drawsContent": true,
               "children": [
                 {
                   "bounds": [285, 300],
+                  "shouldFlattenTransform": false,
                   "children": [
                     {
-                      "bounds": [285, 1000],
-                      "shouldFlattenTransform": false
+                      "bounds": [285, 1000]
                     }
                   ]
                 },
                 {
                   "bounds": [300, 300],
                   "children": [
                     {
                       "position": [285, 0],
                       "bounds": [15, 300]
                     }
@@ skipping 15 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }