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

third_party/WebKit/LayoutTests/compositing/scrollbars/nested-overlay-scrollbars-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
@@ skipping 39 matching lines @@
                         },
                         {
                           "position": [2, 2502],
                           "bounds": [5000, 1000],
                           "drawsContent": true
                         }
                       ]
                     },
                     {
                       "position": [2, 502],
+                      "bounds": [204, 204],
                       "children": [
                         {
                           "position": [2, 195],
                           "bounds": [193, 7]
                         },
                         {
                           "position": [195, 2],
                           "bounds": [7, 193]
                         },
                         {
                           "position": [195, 195],
                           "bounds": [7, 7],
                           "drawsContent": true
                         }
                       ]
                     }
                   ]
                 }
               ]
             },
             {
+              "bounds": [404, 404],
               "children": [
                 {
                   "position": [395, 2],
                   "bounds": [7, 400]
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }