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

third_party/WebKit/LayoutTests/platform/linux/compositing/layer-creation/fixed-position-nonscrollable-body-mismatch-containers-expected.txt

 Even though the fixed-position element's container is nonscrollable, it should still be composited because one of its ancestors is scrolling.
 
 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 72],
           "bounds": [302, 302],
           "drawsContent": true,
           "children": [
             {
               "position": [1, 1],
               "bounds": [285, 285],
               "children": [
                 {
                   "bounds": [285, 800],
                   "shouldFlattenTransform": false,
                   "drawsContent": true
                 }
               ]
             },
             {
+              "bounds": [302, 302],
               "children": [
                 {
                   "position": [1, 286],
                   "bounds": [285, 15]
                 },
                 {
                   "position": [286, 1],
                   "bounds": [15, 285]
                 },
                 {
@@ skipping 10 matching lines @@
           "bounds": [100, 100],
           "contentsOpaque": true,
           "drawsContent": true,
           "backgroundColor": "#00FF00"
         }
       ]
     }
   ]
 }