services/ui: Create a proper component for the client-lib.

services/ui/public/cpp/window_tree_client_observer.h

Index: services/ui/public/cpp/window_tree_client_observer.h
diff --git a/services/ui/public/cpp/window_tree_client_observer.h b/services/ui/public/cpp/window_tree_client_observer.h
index 65f45e1ac2db4efb02977a98a0a41d0ceee49c87..8d42d37503776097c8db32b2088ca7e76149a2b2 100644
--- a/services/ui/public/cpp/window_tree_client_observer.h
+++ b/services/ui/public/cpp/window_tree_client_observer.h
@@ -5,12 +5,14 @@
 #ifndef SERVICES_UI_PUBLIC_CPP_WINDOW_TREE_CLIENT_OBSERVER_H_
 #define SERVICES_UI_PUBLIC_CPP_WINDOW_TREE_CLIENT_OBSERVER_H_
 
+#include "services/ui/public/cpp/ui_export.h"
+
 namespace ui {
 
 class Window;
 class WindowTreeClient;
 
-class WindowTreeClientObserver {
+class UI_EXPORT WindowTreeClientObserver {
  public:
   virtual void OnWindowTreeFocusChanged(Window* gained_focus,
                                         Window* lost_focus) {}