Moves TransientWindowClient to ui/aura/client and adds observer

ui/wm/core/transient_window_controller.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/wm/core/transient_window_controller.h"
 
+#include "ui/aura/client/transient_window_client_observer.h"
 #include "ui/wm/core/transient_window_manager.h"
 
 namespace wm {
 
+// static
+TransientWindowController* TransientWindowController::instance_ = nullptr;
+
 TransientWindowController::TransientWindowController() {
+  DCHECK(!instance_);
+  instance_ = this;
 }
 
 TransientWindowController::~TransientWindowController() {
+  DCHECK_EQ(instance_, this);
+  instance_ = nullptr;
 }
 
 void TransientWindowController::AddTransientChild(aura::Window* parent,
                                                   aura::Window* child) {
   TransientWindowManager::Get(parent)->AddTransientChild(child);
 }
 
 void TransientWindowController::RemoveTransientChild(aura::Window* parent,
                                                      aura::Window* child) {
   TransientWindowManager::Get(parent)->RemoveTransientChild(child);
 }
 
 aura::Window* TransientWindowController::GetTransientParent(
     aura::Window* window) {
   return const_cast<aura::Window*>(GetTransientParent(
       const_cast<const aura::Window*>(window)));
 }
 
 const aura::Window* TransientWindowController::GetTransientParent(
     const aura::Window* window) {
   const TransientWindowManager* window_manager =
       TransientWindowManager::Get(window);
   return window_manager ? window_manager->transient_parent() : NULL;
 }
 
+void TransientWindowController::AddObserver(
+    aura::client::TransientWindowClientObserver* observer) {
+  observers_.AddObserver(observer);
+}
+
+void TransientWindowController::RemoveObserver(
+    aura::client::TransientWindowClientObserver* observer) {
+  observers_.RemoveObserver(observer);
+}
+
 }  // namespace wm