Enable slimming paint for fast/repaint

LayoutTests/platform/android/fast/repaint/resize-scrollable-div-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [8, 108, 300, 300],
         [8, 108, 100, 100],
         [0, 216, 800, 200]
       ],
       "paintInvalidationClients": [
         "HorizontalScrollbar",
         "VerticalScrollbar",
         "VerticalScrollbar",
         "VerticalScrollbar",
         "HorizontalScrollbar",
         "HorizontalScrollbar",
         "LayoutBlockFlow HTML",
         "LayoutBlockFlow DIV id='div'"
       ]
     }
   ]
 }