Fix flattening for scrollable areas.

third_party/WebKit/LayoutTests/virtual/prefer_compositing_to_lcd_text/compositing/overflow/accelerated-overflow-scroll-should-not-affect-perspective-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
           "bounds": [200, 200],
+          "shouldFlattenTransform": false,
           "drawsContent": true,
           "children": [
             {
               "bounds": [200, 200],
               "shouldFlattenTransform": false,
               "transform": [
                 [1, 0, 0, 0],
                 [0, 1, 0, 0],
                 [0, 0, 1, -0.01],
                 [0, 0, 0, 1]
@@ skipping 57 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }