Makes ash/mus use RootWindowControllerCommon

ash/common/root_window_controller_common.h

Index: ash/common/root_window_controller_common.h
diff --git a/ash/common/root_window_controller_common.h b/ash/common/root_window_controller_common.h
index 3386d4838d197b64e68fdb8348197edb56bec1cb..3c26739cce2ab69d43659e3b5d9199e2ce95491d 100644
--- a/ash/common/root_window_controller_common.h
+++ b/ash/common/root_window_controller_common.h
@@ -5,6 +5,7 @@
 #ifndef ASH_COMMON_ROOT_WINDOW_CONTROLLER_COMMON_H_
 #define ASH_COMMON_ROOT_WINDOW_CONTROLLER_COMMON_H_
 
+#include "ash/ash_export.h"
 #include "base/macros.h"
 
 namespace ash {
@@ -16,7 +17,7 @@ class WmWindow;
 // This will eventually become what is RootWindowController. During the
 // transition it contains code used by both the aura and mus implementations.
 // It should *not* contain any aura specific code.
-class RootWindowControllerCommon {
+class ASH_EXPORT RootWindowControllerCommon {
  public:
   explicit RootWindowControllerCommon(wm::WmWindow* root);
   ~RootWindowControllerCommon();