Fix flattening for scrollable areas.

third_party/WebKit/LayoutTests/virtual/prefer_compositing_to_lcd_text/compositing/overflow/reparented-scrollbars-non-sc-anc-expected.txt

 {
   "bounds": [1208, 821],
   "children": [
     {
       "bounds": [1208, 821],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
           "bounds": [1200, 800],
           "shouldFlattenTransform": false,
           "children": [
             {
               "bounds": [1200, 800],
               "drawsContent": true,
               "children": [
                 {
                   "bounds": [1200, 800],
                   "children": [
                     {
                       "bounds": [1200, 1000],
                       "shouldFlattenTransform": false,
                       "children": [
                         {
                           "bounds": [1200, 1000],
+                          "shouldFlattenTransform": false,
                           "drawsContent": true,
                           "children": [
                             {
                               "bounds": [1200, 1000],
+                              "shouldFlattenTransform": false,
                               "children": [
                                 {
                                   "bounds": [1200, 10000],
-                                  "shouldFlattenTransform": false,
                                   "drawsContent": true
                                 }
                               ]
                             }
                           ]
                         }
                       ]
                     }
                   ]
                 }
@@ skipping 18 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }