Fix flattening for scrollable areas.

third_party/WebKit/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],
+          "shouldFlattenTransform": false,
           "drawsContent": true,
           "repaintRects": [
             [-2000, -2000, 5000, 5000]
           ],
           "children": [
             {
               "bounds": [485, 485],
+              "shouldFlattenTransform": false,
               "children": [
                 {
                   "bounds": [5000, 5000],
-                  "shouldFlattenTransform": false,
                   "drawsContent": true,
                   "repaintRects": [
                     [0, 0, 5000, 5000]
                   ],
                   "paintInvalidationClients": [
                     "LayoutBlockFlow DIV id='content'"
                   ]
                 }
               ]
             },
@@ skipping 21 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }