Moves TransientWindowClient to ui/aura/client and adds observer

ui/wm/core/transient_window_controller.cc

Index: ui/wm/core/transient_window_controller.cc
diff --git a/ui/wm/core/transient_window_controller.cc b/ui/wm/core/transient_window_controller.cc
index bb1945c09b2e89e75fe20f54ba3e618cfe39f034..f1fc00d1a012de83c541464f61a8681873ec3ade 100644
--- a/ui/wm/core/transient_window_controller.cc
+++ b/ui/wm/core/transient_window_controller.cc
@@ -4,14 +4,22 @@
 
 #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,
@@ -37,4 +45,14 @@ const aura::Window* TransientWindowController::GetTransientParent(
   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