Deflake repaint layout tests which assumed scroll would happen before first paint.

LayoutTests/fast/repaint/vertical-overflow-parent.html

Index: LayoutTests/fast/repaint/vertical-overflow-parent.html
diff --git a/LayoutTests/fast/repaint/vertical-overflow-parent.html b/LayoutTests/fast/repaint/vertical-overflow-parent.html
index 92a7949c31b2c94cc322293702ddfef744e5f8cd..912c02495df692447e9a1b9c9c61e5ed4cdb4059 100644
--- a/LayoutTests/fast/repaint/vertical-overflow-parent.html
+++ b/LayoutTests/fast/repaint/vertical-overflow-parent.html
@@ -38,7 +38,9 @@
             target.style.visibility = "visible";
         };
         var container = document.getElementsByClassName("container")[0];
-        container.scrollLeft = 0;
-        runRepaintTest();
+        runAfterDisplay(function() {
+            container.scrollLeft = 0;
+            runRepaintTest();
+        });
     </script>
 </body></html>