Reduce invalidation on children-needs-layout containers

LayoutTests/fast/box-shadow/negative-shadow-box-shrink-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [100, 200, 640, 100],
-        [100, 200, 640, 100],
         [100, 200, 600, 100],
-        [100, 160, 640, 40],
-        [0, 300, 800, 100]
+        [100, 160, 640, 40]
       ]
     }
   ]
 }