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

third_party/WebKit/LayoutTests/compositing/overflow/resize-painting-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [18, 10],
           "bounds": [100, 100],
           "drawsContent": true,
           "children": [
             {
-              "position": [85, 85],
-              "bounds": [15, 15],
-              "drawsContent": true
+              "bounds": [100, 100],
+              "children": [
+                {
+                  "position": [85, 85],
+                  "bounds": [15, 15],
+                  "drawsContent": true
+                }
+              ]
             }
           ]
         }
       ]
     }
   ]
 }