Enable slimming paint for fast/repaint

LayoutTests/platform/mac/fast/repaint/line-flow-with-floats-6-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [27, 353, 22, 18],
         [14, 353, 48, 64],
         [8, 548, 418, 18]
+      ],
+      "paintInvalidationClients": [
+        "InlineTextBox 'here; the great\n'",
+        "InlineTextBox 'wonder is, that there\u2018s any one left alive!\u2019'",
+        "RootInlineBox",
+        "LayoutBlockFlow P",
+        "LayoutBlockFlow (floating) SPAN id='blueFloat'",
+        "LayoutText #text",
+        "InlineTextBox 'foo'"
       ]
     }
   ]
 }