ash/mash: Add a mus-client that sets up ash to provide the system ui for mash.

ash/root_window_settings.h

Index: ash/root_window_settings.h
diff --git a/ash/root_window_settings.h b/ash/root_window_settings.h
index 84abc3fb247d3e567d180feb153ee23400282506..65a70d58392601362557aa99ee196adf0196ceb8 100644
--- a/ash/root_window_settings.h
+++ b/ash/root_window_settings.h
@@ -33,7 +33,7 @@ struct RootWindowSettings {
 
 // Initializes and returns RootWindowSettings for |root|.
 // It is owned by the |root|.
-RootWindowSettings* InitRootWindowSettings(aura::Window* root);
+ASH_EXPORT RootWindowSettings* InitRootWindowSettings(aura::Window* root);
 
 // Returns the RootWindowSettings for |root|.
 ASH_EXPORT RootWindowSettings* GetRootWindowSettings(aura::Window* root);