Make sure we set "should flatten transform" correctly.

LayoutTests/platform/linux/virtual/softwarecompositing/overflow/selection-gaps-after-removing-scrolling-contents-expected.txt

 This test passes if the container's scrolling contents layer (the first child of the GraphicsLayer with 4 children) draws content only on iterations 1 and 2, and its scrolling block selection layer (the child of the scrolling contents layer) draws content only on iteration 3. The scrolling block selection layer should also be much smaller than the scrolling contents layer.
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@@ skipping 19 matching lines @@
           "children": [
             {
               "bounds": [285, 485],
               "children": [
                 {
                   "bounds": [285, 665],
                   "shouldFlattenTransform": false,
                   "drawsContent": true,
                   "children": [
                     {
-
+                      "shouldFlattenTransform": false
                     }
                   ]
                 }
               ]
             },
             {
               "children": [
                 {
                   "position": [0, 485],
                   "bounds": [285, 15],
@@ skipping 108 matching lines @@
           "children": [
             {
               "bounds": [285, 485],
               "children": [
                 {
                   "bounds": [285, 665],
                   "shouldFlattenTransform": false,
                   "drawsContent": true,
                   "children": [
                     {
-
+                      "shouldFlattenTransform": false
                     }
                   ]
                 }
               ]
             },
             {
               "children": [
                 {
                   "position": [0, 485],
                   "bounds": [285, 15],
@@ skipping 106 matching lines @@
             {
               "bounds": [285, 485],
               "children": [
                 {
                   "bounds": [285, 665],
                   "shouldFlattenTransform": false,
                   "children": [
                     {
                       "position": [0, 195],
                       "bounds": [285, 15],
+                      "shouldFlattenTransform": false,
                       "drawsContent": true
                     }
                   ]
                 }
               ]
             },
             {
               "children": [
                 {
                   "position": [0, 485],
@@ skipping 86 matching lines @@
         }
       ]
     }
   ]
 }
 
 
 *** iteration 4: ***
 
 {
-  "bounds": [785, 6754],
+  "bounds": [785, 6770],
   "children": [
     {
-      "bounds": [785, 6754],
+      "bounds": [785, 6770],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 68],
           "bounds": [300, 500],
           "children": [
             {
               "bounds": [285, 485],
               "children": [
                 {
                   "bounds": [285, 665],
                   "shouldFlattenTransform": false,
                   "children": [
                     {
                       "position": [0, 195],
-                      "bounds": [285, 15]
+                      "bounds": [285, 15],
+                      "shouldFlattenTransform": false
                     }
                   ]
                 }
               ]
             },
             {
               "children": [
                 {
                   "position": [0, 485],
                   "bounds": [285, 15],
@@ skipping 81 matching lines @@
           "bounds": [100, 50],
           "contentsOpaque": true,
           "drawsContent": true,
           "backgroundColor": "#FFA500"
         }
       ]
     }
   ]
 }