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

third_party/WebKit/LayoutTests/compositing/overflow/composited-scrolling-paint-phases-expected.txt

 {
   "bounds": [800, 600],
   "paintingPhases": [
     "GraphicsLayerPaintBackground",
     "GraphicsLayerPaintForeground",
     "GraphicsLayerPaintMask"
   ],
   "children": [
     {
       "bounds": [800, 600],
@@ skipping 56 matching lines @@
                       "drawsContent": true,
                       "paintingPhases": [
                         "GraphicsLayerPaintForeground",
                         "GraphicsLayerPaintOverflowContents",
                         "GraphicsLayerPaintCompositedScroll"
                       ]
                     }
                   ]
                 },
                 {
+                  "bounds": [202, 202],
                   "paintingPhases": [
                     "GraphicsLayerPaintBackground",
                     "GraphicsLayerPaintForeground",
                     "GraphicsLayerPaintMask"
                   ],
                   "children": [
                     {
                       "position": [1, 186],
                       "bounds": [185, 15],
                       "paintingPhases": [
@@ skipping 26 matching lines @@
               ]
             }
           ]
         }
       ]
     }
   ]
 }