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

third_party/WebKit/LayoutTests/compositing/squashing/composited-bounds-for-negative-z-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "children": [
         {
           "bounds": [800, 408],
           "drawsContent": true,
@@ skipping 17 matching lines @@
                 {
                   "bounds": [285, 300],
                   "children": [
                     {
                       "bounds": [285, 1000],
                       "shouldFlattenTransform": false
                     }
                   ]
                 },
                 {
+                  "bounds": [300, 300],
                   "children": [
                     {
                       "position": [285, 0],
                       "bounds": [15, 300]
                     }
                   ]
                 }
               ]
             },
             {
@@ skipping 10 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }