Simplify application of perspective origin.

LayoutTests/platform/win/compositing/squashing/composited-bounds-for-negative-z-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "children": [
         {
           "bounds": [800, 408],
           "drawsContent": true,
@@ skipping 12 matching lines @@
             {
               "position": [108, 100],
               "bounds": [300, 300],
               "drawsContent": true,
               "children": [
                 {
                   "bounds": [285, 300],
                   "children": [
                     {
                       "bounds": [285, 1000],
-                      "shouldFlattenTransform": false,
                       "children": [
                         {
-                          "shouldFlattenTransform": false
+
                         }
                       ]
                     }
                   ]
                 },
                 {
                   "children": [
                     {
                       "position": [285, 0],
                       "bounds": [15, 300],
@@ skipping 17 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }