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

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

Index: LayoutTests/fast/repaint/horizontal-bt-overflow-same.html
diff --git a/LayoutTests/fast/repaint/horizontal-bt-overflow-same.html b/LayoutTests/fast/repaint/horizontal-bt-overflow-same.html
index ca19f7782b2d26687828109d735553c04f66f049..f8e44c1b08aa682b95e118ae7706220c9b7673df 100644
--- a/LayoutTests/fast/repaint/horizontal-bt-overflow-same.html
+++ b/LayoutTests/fast/repaint/horizontal-bt-overflow-same.html
@@ -37,7 +37,9 @@
             target.style.visibility = "visible";
         };
         var container = document.getElementsByClassName("container")[0];
-        container.scrollTop = 0;
-        runRepaintTest();
+        runAfterDisplay(function() {
+            container.scrollTop = 0;
+            runRepaintTest();
+        });
     </script>
 </body></html>