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

third_party/WebKit/LayoutTests/compositing/layer-creation/fixed-position-in-fixed-overflow-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 13],
@@ skipping 13 matching lines @@
                       "bounds": [100, 100],
                       "contentsOpaque": true,
                       "drawsContent": true,
                       "backgroundColor": "#FFFF00"
                     }
                   ]
                 }
               ]
             },
             {
+              "bounds": [800, 600],
               "children": [
                 {
                   "position": [785, 0],
                   "bounds": [15, 600]
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }