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

third_party/WebKit/LayoutTests/compositing/overflow/scrolling-without-painting-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
           "bounds": [202, 202],
           "drawsContent": true,
           "children": [
             {
               "position": [1, 1],
               "bounds": [185, 185],
               "children": [
                 {
                   "bounds": [185, 1025],
                   "shouldFlattenTransform": false,
                   "drawsContent": true
                 }
               ]
             },
             {
+              "bounds": [202, 202],
               "children": [
                 {
                   "position": [1, 186],
                   "bounds": [185, 15]
                 },
                 {
                   "position": [186, 1],
                   "bounds": [15, 185],
                   "repaintRects": [
                     [0, 0, 15, 185]
                   ]
                 },
                 {
                   "position": [186, 186],
                   "bounds": [15, 15],
                   "drawsContent": true
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }