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

third_party/WebKit/LayoutTests/fast/repaint/overflow-move-after-scroll-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [10, 60],
@@ skipping 15 matching lines @@
                     [300, 200, 120, 50],
                     [50, 200, 120, 50]
                   ],
                   "paintInvalidationClients": [
                     "LayoutBlockFlow (positioned) DIV id='block'"
                   ]
                 }
               ]
             },
             {
+              "bounds": [700, 400],
               "children": [
                 {
                   "position": [0, 385],
                   "bounds": [685, 15]
                 },
                 {
                   "position": [685, 0],
                   "bounds": [15, 385],
                   "repaintRects": [
                     [0, 0, 15, 385]
                   ]
                 },
                 {
                   "position": [685, 385],
                   "bounds": [15, 15],
                   "drawsContent": true
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }