Enable slimming paint for fast/repaint

LayoutTests/fast/repaint/filter-repaint-accelerated-child-with-filter-child-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [-20, -20],
           "bounds": [256, 256],
           "drawsContent": true,
           "children": [
             {
               "position": [16, 16],
               "transformOrigin": [112, 112],
               "bounds": [212, 256],
               "drawsContent": true,
               "repaintRects": [
                 [0, 0, 256, 256],
                 [0, 0, 156, 256]
+              ],
+              "paintInvalidationClients": [
+                "LayoutBlockFlow DIV id='resize' class='drop-shadow'"
               ]
             }
           ]
         }
       ]
     }
   ]
 }