Enable slimming paint for fast/repaint

LayoutTests/platform/linux/fast/repaint/offset-change-wrong-invalidation-with-float-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [70, 94, 18, 20],
         [70, 94, 7, 19],
         [70, -156, 18, 20],
         [70, -156, 7, 19],
         [48, 94, 40, 20],
         [48, -156, 40, 20]
       ],
+      "paintInvalidationClients": [
+        "LayoutBlockFlow (positioned) UL id='submenu'",
+        "LayoutListItem LI",
+        "LayoutListMarker (anonymous)"
+      ],
       "children": [
         {
           "position": [30, 44],
           "transformOrigin": [18, 10],
           "bounds": [25, 20],
           "drawsContent": true,
           "children": [
             {
               "position": [18, 0],
               "bounds": [0, 19],
               "backfaceVisibility": "hidden"
             }
           ]
         },
         {
           "position": [30, 44],
           "bounds": [18, 20],
           "drawsContent": true
         }
       ]
     }
   ]
 }