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

third_party/WebKit/LayoutTests/compositing/update-paint-phases-expected.txt

 {
   "bounds": [800, 600],
   "paintingPhases": [
     "GraphicsLayerPaintBackground",
     "GraphicsLayerPaintForeground",
     "GraphicsLayerPaintMask"
   ],
   "children": [
     {
       "bounds": [800, 600],
@@ skipping 30 matching lines @@
                   "drawsContent": true,
                   "paintingPhases": [
                     "GraphicsLayerPaintForeground",
                     "GraphicsLayerPaintOverflowContents",
                     "GraphicsLayerPaintCompositedScroll"
                   ]
                 }
               ]
             },
             {
+              "bounds": [102, 102],
               "paintingPhases": [
                 "GraphicsLayerPaintBackground",
                 "GraphicsLayerPaintForeground",
                 "GraphicsLayerPaintMask"
               ],
               "children": [
                 {
                   "position": [1, 86],
                   "bounds": [85, 15],
                   "paintingPhases": [
@@ skipping 23 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }