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

third_party/WebKit/LayoutTests/paint/invalidation/invalidate-after-composited-scroll-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [300, 300],
@@ skipping 13 matching lines @@
                   "repaintRects": [
                     [0, 2400, 100, 100]
                   ],
                   "paintInvalidationClients": [
                     "LayoutBlockFlow DIV id='target'"
                   ]
                 }
               ]
             },
             {
+              "bounds": [200, 200],
               "children": [
                 {
                   "position": [185, 0],
                   "bounds": [15, 200],
                   "repaintRects": [
                     [0, 0, 15, 200]
                   ]
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }