Fix flattening for scrollable areas.

third_party/WebKit/LayoutTests/paint/invalidation/invalidate-after-composited-scroll-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [300, 300],
           "bounds": [200, 200],
+          "shouldFlattenTransform": false,
           "drawsContent": true,
           "repaintRects": [
             [0, 50, 100, 100]
           ],
           "children": [
             {
               "bounds": [185, 200],
+              "shouldFlattenTransform": false,
               "children": [
                 {
                   "bounds": [185, 4900],
-                  "shouldFlattenTransform": false,
                   "drawsContent": true,
                   "repaintRects": [
                     [0, 2400, 100, 100]
                   ],
                   "paintInvalidationClients": [
                     "LayoutBlockFlow DIV id='target'"
                   ]
                 }
               ]
             },
             {
               "bounds": [200, 200],
               "children": [
                 {
                   "position": [185, 0],
                   "bounds": [15, 200],
                   "repaintRects": [
                     [0, 0, 15, 200]
                   ]
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }