Centralizes more shared code between ash and mash

ash/mus/test/wm_test_helper.h

Index: ash/mus/test/wm_test_helper.h
diff --git a/ash/mus/test/wm_test_helper.h b/ash/mus/test/wm_test_helper.h
index 88e38d7b1fa07266f4ddf59e04f67a48db868e4c..6e7353daeff049a36715159bfd1a136f0ba252c9 100644
--- a/ash/mus/test/wm_test_helper.h
+++ b/ash/mus/test/wm_test_helper.h
@@ -53,11 +53,13 @@ class WmTestHelper {
   // Creates a new RootWindowController given |display_spec|, which is the
   // configuration of the display.

[James Cook, 2016/09/21 22:03:19]

nit: Document |next_x| here and below.

Or consider making it a member.

[sky, 2016/09/21 22:47:49]

Done.

   RootWindowController* CreateRootWindowController(
-      const std::string& display_spec);
+      const std::string& display_spec,
+      int* next_x);
 
   // Updates the display of an existing RootWindowController.
   void UpdateDisplay(RootWindowController* root_window_controller,
-                     const std::string& display_spec);
+                     const std::string& display_spec,
+                     int* next_x);
 
   // Destroys a RootWindowController.
   void DestroyRootWindowController(