Enable slimming paint for fast/repaint

LayoutTests/platform/linux/fast/repaint/scroll-stacking-context-backface-visiblity-leaves-traces-expected.txt

 {
   "bounds": [785, 5056],
   "children": [
     {
       "bounds": [785, 5056],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 28],
           "bounds": [769, 1972],
           "drawsContent": true,
           "backfaceVisibility": "hidden",
           "repaintRects": [
             [10, 1822, 150, 150],
             [10, 1822, 150, 150],
             [10, 1422, 150, 150],
             [10, 1422, 150, 150]
+          ],
+          "paintInvalidationClients": [
+            "LayoutBlockFlow (positioned) DIV id='searchbar'",
+            "LayoutBlockFlow DIV id='recentlink'"
           ]
         }
       ]
     }
   ]
 }