Simplify application of perspective origin.

LayoutTests/platform/linux/compositing/repaint/should-not-clip-composited-overflow-scrolling-layer-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
           "bounds": [500, 500],
           "drawsContent": true,
           "repaintRects": [
             [0, 20, 821, 19],
             [0, 0, 5000, 5000],
             [0, 0, 801, 19],
             [-2000, -1980, 821, 19],
             [-2000, -2000, 5000, 5000],
             [-2000, -2000, 801, 19]
           ],
           "children": [
             {
               "bounds": [485, 485],
               "children": [
                 {
                   "bounds": [5000, 5000],
-                  "shouldFlattenTransform": false,
                   "drawsContent": true,
                   "repaintRects": [
                     [2000, 2020, 821, 19],
                     [2000, 2000, 5000, 5000],
                     [2000, 2000, 801, 19],
                     [0, 20, 821, 19],
                     [0, 0, 5000, 5000],
                     [0, 0, 801, 19]
                   ],
                   "children": [
                     {
-                      "shouldFlattenTransform": false
+
                     }
                   ]
                 }
               ]
             },
             {
               "children": [
                 {
                   "position": [0, 485],
                   "bounds": [485, 15],
@@ skipping 17 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }