Enable slimming paint for fast/repaint

LayoutTests/platform/android/fast/repaint/video-unmute-repaint-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
           "bounds": [700, 525]
         },
         {
+          "shouldFlattenTransform": false,
           "children": [
             {
               "position": [8, 8],
               "bounds": [700, 525],
               "drawsContent": true
             },
             {
               "position": [8, 8],
               "bounds": [700, 520],
               "drawsContent": true,
               "repaintRects": [
                 [599, 493, 24, 24],
                 [570, 493, 89, 24],
                 [570, 493, 24, 24],
                 [535, 490, 35, 30]
               ]
             }
           ]
         }
       ]
     }
   ]
 }