Make sure we set "should flatten transform" correctly.

LayoutTests/platform/linux/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],
           "children": [
             {
               "bounds": [185, 185],
               "shouldFlattenTransform": false,
               "transform": [
                 [1, 0, 0, 0],
                 [0, 1, 0, 0],
                 [0, 0, 1, -0.01],
                 [0, 0, 0, 1]
               ],
               "children": [
                 {
                   "bounds": [185, 265],
                   "shouldFlattenTransform": false,
                   "children": [
                     {
+                      "shouldFlattenTransform": false,
                       "children": [
                         {
                           "position": [0, 65],
                           "bounds": [60, 200],
                           "contentsOpaque": true,
                           "drawsContent": true,
                           "backgroundColor": "#008000",
                           "transform": [
                             [1, 0, 0, 0],
                             [0, 1, 0, 0],
@@ skipping 39 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }