Let synchronized painting generate correct paint invalidation rects

third_party/WebKit/LayoutTests/fast/repaint/scrollbar-damage-and-full-viewport-repaint-expected.txt

 {
   "bounds": [1000, 600],
   "children": [
     {
       "bounds": [1000, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [101, 51, 101, 201],
         [1, 236, 185, 15]
+      ],
+      "paintInvalidationClients": [
+        "HorizontalScrollbar",
+        "LayoutBlockFlow DIV id='child'"
       ]
     }
   ]
 }