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

LayoutTests/fast/repaint/horizontal-bt-overflow-parent.html

Index: LayoutTests/fast/repaint/horizontal-bt-overflow-parent.html
diff --git a/LayoutTests/fast/repaint/horizontal-bt-overflow-parent.html b/LayoutTests/fast/repaint/horizontal-bt-overflow-parent.html
index f2aecb7b95a7d91b3b2d1d7c12f36ab3625feffc..89743ad3367dc970e3c8bbce810d815a235ab31f 100644
--- a/LayoutTests/fast/repaint/horizontal-bt-overflow-parent.html
+++ b/LayoutTests/fast/repaint/horizontal-bt-overflow-parent.html
@@ -38,7 +38,9 @@
             target.style.visibility = "visible";
         };
         var container = document.getElementsByClassName("container")[0];
-        container.scrollTop = 0;
-        runRepaintTest();
+        runAfterDisplay(function() {
+            container.scrollTop = 0;
+            runRepaintTest();
+        });
     </script>
 </body></html>