Moves TransientWindowClient to ui/aura/client and adds observer

ui/wm/core/transient_window_controller.h

Index: ui/wm/core/transient_window_controller.h
diff --git a/ui/wm/core/transient_window_controller.h b/ui/wm/core/transient_window_controller.h
index 2ba28bb778041ad75fdf73c464efa9d2970c7370..1f7a0c42edc43d58aaa18566ad608f4bed58755a 100644
--- a/ui/wm/core/transient_window_controller.h
+++ b/ui/wm/core/transient_window_controller.h
@@ -6,11 +6,14 @@
 #define UI_WM_CORE_TRANSIENT_WINDOW_CONTROLLER_H_
 
 #include "base/macros.h"
-#include "ui/wm/public/transient_window_client.h"
+#include "base/observer_list.h"
+#include "ui/aura/client/transient_window_client.h"
 #include "ui/wm/wm_export.h"
 
 namespace wm {
 
+class TransientWindowManager;
+
 // TransientWindowClient implementation. Uses TransientWindowManager to handle
 // tracking transient per window.
 class WM_EXPORT TransientWindowController
@@ -19,13 +22,26 @@ class WM_EXPORT TransientWindowController
   TransientWindowController();
   ~TransientWindowController() override;
 
+  // Returns the single TransientWindowController instance.
+  static TransientWindowController* Get() { return instance_; }
+
   // TransientWindowClient:
   void AddTransientChild(aura::Window* parent, aura::Window* child) override;
   void RemoveTransientChild(aura::Window* parent, aura::Window* child) override;
   aura::Window* GetTransientParent(aura::Window* window) override;
   const aura::Window* GetTransientParent(const aura::Window* window) override;
+  void AddObserver(
+      aura::client::TransientWindowClientObserver* observer) override;
+  void RemoveObserver(
+      aura::client::TransientWindowClientObserver* observer) override;
 
  private:
+  friend class TransientWindowManager;
+
+  static TransientWindowController* instance_;
+
+  base::ObserverList<aura::client::TransientWindowClientObserver> observers_;
+
   DISALLOW_COPY_AND_ASSIGN(TransientWindowController);
 };