Enable slimming paint for fast/repaint

LayoutTests/platform/android/fast/repaint/stacking-context-lost-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [0, 0, 100, 100]
       ],
       "children": [
         {
           "position": [278, 278],
           "bounds": [100, 100],
           "contentsOpaque": true,
           "drawsContent": true,
           "backfaceVisibility": "hidden",
           "backgroundColor": "#008000",
           "repaintRects": [
             [0, 0, 100, 100],
             [0, 0, 100, 100]
           ]
         }
       ]
     }
   ]
 }