Implement hit tests/client area.

ui/views/mus/mus_client.h

Index: ui/views/mus/mus_client.h
diff --git a/ui/views/mus/mus_client.h b/ui/views/mus/mus_client.h
index 38087cd9e7815e036d03f56023832850e54a30e0..b2bb69239ad029b7036d28b7d0c477148d956918 100644
--- a/ui/views/mus/mus_client.h
+++ b/ui/views/mus/mus_client.h
@@ -40,6 +40,7 @@ class WMState;
 
 namespace views {
 
+class MusClientObserver;
 class ScreenMus;
 
 namespace internal {
@@ -82,6 +83,9 @@ class VIEWS_MUS_EXPORT MusClient
   NativeWidget* CreateNativeWidget(const Widget::InitParams& init_params,
                                    internal::NativeWidgetDelegate* delegate);
 
+  void AddObserver(MusClientObserver* observer);
+  void RemoveObserver(MusClientObserver* observer);
+
  private:
   friend class AuraInit;
   friend class test::MusClientTestApi;
@@ -114,6 +118,8 @@ class VIEWS_MUS_EXPORT MusClient
   service_manager::Connector* connector_;
   service_manager::Identity identity_;
 
+  base::ObserverList<MusClientObserver> observer_list_;
+
   std::unique_ptr<wm::WMState> wm_state_;
 
   std::unique_ptr<ScreenMus> screen_;