Updates shadow when active window changes

components/mus/public/cpp/lib/window_tree_client_impl.h

Index: components/mus/public/cpp/lib/window_tree_client_impl.h
diff --git a/components/mus/public/cpp/lib/window_tree_client_impl.h b/components/mus/public/cpp/lib/window_tree_client_impl.h
index af224dbc0e1b6fbbe3b438aed90771c4a4cd53ee..7a40c4468c5f1d03c07727e687e25bcbfe91aea6 100644
--- a/components/mus/public/cpp/lib/window_tree_client_impl.h
+++ b/components/mus/public/cpp/lib/window_tree_client_impl.h
@@ -7,6 +7,7 @@
 
 #include <map>
 
+#include "base/observer_list.h"
 #include "components/mus/common/types.h"
 #include "components/mus/public/cpp/window.h"
 #include "components/mus/public/cpp/window_tree_connection.h"
@@ -129,6 +130,8 @@ class WindowTreeClientImpl : public WindowTreeConnection,
   Window* NewWindow(const Window::SharedProperties* properties) override;
   bool IsEmbedRoot() override;
   ConnectionSpecificId GetConnectionId() override;
+  void AddObserver(WindowTreeConnectionObserver* observer) override;
+  void RemoveObserver(WindowTreeConnectionObserver* observer) override;
 
   // Overridden from WindowTreeClient:
   void OnEmbed(ConnectionSpecificId connection_id,
@@ -211,6 +214,8 @@ class WindowTreeClientImpl : public WindowTreeConnection,
 
   bool in_destructor_;
 
+  base::ObserverList<WindowTreeConnectionObserver> observers_;
+
   MOJO_DISALLOW_COPY_AND_ASSIGN(WindowTreeClientImpl);
 };