Simplify application of perspective origin.

LayoutTests/platform/linux/virtual/slimmingpaint/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],
@@ skipping 17 matching lines @@
             "LayoutText #text",
             "InlineTextBox 'Note for manual testing: must run with --enable-prefer-compositing-to-lcd-text\n'",
             "InlineTextBox 'on non-high-dpi machines to enable composited scrolling.'"
           ],
           "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]
                   ],
                   "paintInvalidationClients": [
                     "LayoutBlockFlow DIV id='content'",
                     "LayoutText #text",
                     "InlineTextBox 'Tests invalidation of scrolling layer. Passes if the repaint rect is not clipped,\n'",
                     "InlineTextBox 'and there is no red when the scrolling container is scrolled.'",
                     "LayoutBR BR",
                     "InlineTextBox '\n'",
                     "LayoutText #text",
                     "InlineTextBox 'Note for manual testing: must run with --enable-prefer-compositing-to-lcd-text\n'",
                     "InlineTextBox 'on non-high-dpi machines to enable composited scrolling.'"
                   ],
                   "children": [
                     {
-                      "shouldFlattenTransform": false
+
                     }
                   ]
                 }
               ]
             },
             {
               "children": [
                 {
                   "position": [0, 485],
                   "bounds": [485, 15],
@@ skipping 23 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }