Clip scrollbars to box bounds when they don't fit.

third_party/WebKit/LayoutTests/compositing/iframes/resizer-expected.txt

 
 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
@@ skipping 33 matching lines @@
                     }
                   ]
                 },
                 {
                   "position": [285, 0],
                   "bounds": [15, 150]
                 }
               ]
             },
             {
-              "position": [287, 137],
-              "bounds": [15, 15],
-              "drawsContent": true
+              "bounds": [304, 154],
+              "children": [
+                {
+                  "position": [287, 137],
+                  "bounds": [15, 15],
+                  "drawsContent": true
+                }
+              ]
             }
           ]
         }
       ]
     }
   ]
 }
 
 
 --------
@@ skipping 12 matching lines @@
           "bounds": [210, 210],
           "contentsOpaque": true,
           "drawsContent": true,
           "backgroundColor": "#0000FF"
         }
       ]
     }
   ]
 }