Apply both phsical bounding box and clip to composited bounds.

third_party/WebKit/LayoutTests/compositing/overflow/reparented-scrollbars-non-sc-anc-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [1208, 821],
       "contentsOpaque": true,
       "drawsContent": true
     },
     {
       "name": "Ancestor Clipping Layer",
@@ skipping 27 matching lines @@
       "shouldFlattenTransform": false
     },
     {
       "name": "Scrolling Contents Layer",
       "bounds": [1200, 10000],
       "drawsContent": true
     },
     {
       "name": "LayoutBlockFlow (relative positioned) DIV id='foreground'",
       "position": [8, 10008],
-      "bounds": [1200, 0]
+      "contentsOpaque": true
     },
     {
       "name": "Overflow Controls Ancestor Clipping Layer",
       "position": [8, 8],
       "bounds": [1200, 1000]
     },
     {
       "name": "Overflow Controls Host Layer",
       "bounds": [1200, 1000]
     },
     {
       "name": "Vertical Scrollbar Layer",
       "position": [1193, 0],
       "bounds": [7, 1000]
     }
   ]
 }