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

third_party/WebKit/LayoutTests/compositing/overflow/reparented-scrollbars-non-sc-anc-expected.txt

 {
   "bounds": [1208, 821],
   "children": [
     {
       "bounds": [1208, 821],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
@@ skipping 37 matching lines @@
         },
         {
           "position": [8, 10008],
           "bounds": [1200, 0]
         },
         {
           "position": [8, 8],
           "bounds": [1200, 1000],
           "children": [
             {
+              "bounds": [1200, 1000],
               "children": [
                 {
                   "position": [1193, 0],
                   "bounds": [7, 1000]
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }