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

third_party/WebKit/LayoutTests/compositing/overflow/accelerated-overflow-scroll-should-not-affect-perspective-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
@@ skipping 44 matching lines @@
                             [0, 0, 20, 1]
                           ]
                         }
                       ]
                     }
                   ]
                 }
               ]
             },
             {
+              "bounds": [200, 200],
               "children": [
                 {
                   "position": [0, 185],
                   "bounds": [185, 15]
                 },
                 {
                   "position": [185, 0],
                   "bounds": [15, 185]
                 },
                 {
                   "position": [185, 185],
                   "bounds": [15, 15],
                   "drawsContent": true
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }