Enable slimming paint for fast/repaint

LayoutTests/platform/android/fast/repaint/fixed-under-composited-fixed-scrolled-expected.txt

 {
   "bounds": [785, 2016],
   "children": [
     {
       "bounds": [785, 2016],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [0, 400],
           "bounds": [785, 600],
           "contentsOpaque": true,
           "drawsContent": true,
           "backfaceVisibility": "hidden",
           "backgroundColor": "#008000",
           "repaintRects": [
             [100, 100, 200, 200]
           ]
         }
       ]
     }
   ]
 }