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

third_party/WebKit/LayoutTests/compositing/overflow/scrolling-content-clip-to-viewport-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "bounds": [320, 340],
           "drawsContent": true,
           "children": [
             {
               "bounds": [305, 325],
               "children": [
                 {
                   "bounds": [305, 1224],
                   "shouldFlattenTransform": false
                 }
               ]
             },
             {
+              "bounds": [320, 340],
               "children": [
                 {
                   "position": [0, 325],
                   "bounds": [305, 15]
                 },
                 {
                   "position": [305, 0],
                   "bounds": [15, 325]
                 },
                 {
@@ skipping 10 matching lines @@
           "bounds": [284, 1204],
           "contentsOpaque": true,
           "drawsContent": true,
           "backgroundColor": "#C0C0C0"
         }
       ]
     }
   ]
 }