Fix visual rect for background box painting in composited scrollers.

third_party/WebKit/LayoutTests/paint/invalidation/composited-overflow-with-local-background-expected.txt

 {
   "name": "Content Root Layer",
   "bounds": [800, 600],
   "children": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
@@ skipping 54 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ],
   "objectPaintInvalidations": [
     {
+      "object": "Scrolling Contents Layer",
+      "reason": "style change"
+    },
+    {
       "object": "LayoutBlockFlow DIV id='scroller'",
       "reason": "style change"
     }
   ]
 }