Apply both phsical bounding box and clip to composited bounds.

third_party/WebKit/LayoutTests/paint/invalidation/compositing/should-not-repaint-composited-descendants-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true
     },
     {
       "name": "LayoutBlockFlow (positioned) DIV id='composited-box'",
       "position": [38, 38],
-      "bounds": [90, 90],
+      "bounds": [20, 70],
+      "contentsOpaque": true,
       "drawsContent": true,
       "backfaceVisibility": "hidden",
       "backgroundColor": "#008000",
       "paintInvalidations": [
         {
           "object": "LayoutBlockFlow (positioned) DIV id='composited-box'",
-          "rect": [0, 0, 70, 70],
-          "reason": "full"
-        },
-        {
-          "object": "LayoutBlockFlow (positioned) DIV id='composited-box'",
           "rect": [-30, -30, 50, 100],
           "reason": "subtree"
         },
         {
           "object": "LayoutBlockFlow DIV class='child'",
           "rect": [-30, -30, 50, 50],
           "reason": "subtree"
+        },
+        {
+          "object": "LayoutBlockFlow (positioned) DIV id='composited-box'",
+          "rect": [0, 0, 10, 60],
+          "reason": "full"
         }
       ]
     },
     {
       "name": "Child Containment Layer",
       "bounds": [90, 90]
     },
     {
       "name": "LayoutBlockFlow DIV class='composited child'",
       "position": [-30, 20],
       "bounds": [50, 50],
       "contentsOpaque": true,
       "drawsContent": true,
       "backfaceVisibility": "hidden",
       "backgroundColor": "#008000"
     }
   ],
   "objectPaintInvalidations": [
     {
       "object": "LayoutBlockFlow (positioned) DIV id='composited-box'",
       "reason": "subtree"
     },
     {
       "object": "LayoutBlockFlow DIV class='child'",
       "reason": "subtree"
     }
   ]
 }