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

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

Index: LayoutTests/fast/repaint/vertical-overflow-child.html
diff --git a/LayoutTests/fast/repaint/vertical-overflow-child.html b/LayoutTests/fast/repaint/vertical-overflow-child.html
index d2d188784e2d324190bf7333a8fccb21b5aa2122..7c2a2a07cea5bcb87f6df031594f558c3a24f8b4 100644
--- a/LayoutTests/fast/repaint/vertical-overflow-child.html
+++ b/LayoutTests/fast/repaint/vertical-overflow-child.html
@@ -38,7 +38,9 @@
             target.style.visibility = "visible";
         };
         var container = document.getElementsByClassName("container")[0];
-        container.scrollLeft = 3000;
-        runRepaintTest();
+        runAfterDisplay(function() {
+            container.scrollLeft = 3000;
+            runRepaintTest();
+        });
     </script>
 </body></html>