Fix flattening for scrollable areas.

third_party/WebKit/LayoutTests/platform/linux/virtual/prefer_compositing_to_lcd_text/compositing/overflow/text-match-highlight-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [259, 40, 40, 19]
       ],
       "paintInvalidationClients": [
         "LayoutText #text",
         "InlineTextBox 'findme'"
       ],
       "children": [
         {
           "position": [0, 60],
           "bounds": [800, 500],
+          "shouldFlattenTransform": false,
           "drawsContent": true,
           "repaintRects": [
             [244, 0, 40, 19],
             [204, 0, 40, 19],
             [83, 0, 40, 19],
             [47, 20, 40, 19],
             [20, 164, 202, 72],
             [10, 131, 138, 12],
             [10, 75, 208, 19]
           ],
           "children": [
             {
               "bounds": [785, 485],
+              "shouldFlattenTransform": false,
               "children": [
                 {
                   "bounds": [785, 1345],
-                  "shouldFlattenTransform": false,
                   "drawsContent": true,
                   "repaintRects": [
                     [244, 0, 40, 19],
                     [204, 0, 40, 19],
                     [83, 0, 40, 19],
                     [47, 20, 40, 19],
                     [20, 164, 202, 72],
                     [10, 131, 138, 12],
                     [10, 75, 208, 19]
                   ],
@@ skipping 34 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }