session_manager: Add SessionManagerObserver interface

components/session_manager/core/session_manager.h

Index: components/session_manager/core/session_manager.h
diff --git a/components/session_manager/core/session_manager.h b/components/session_manager/core/session_manager.h
index 02b7589a4576908d65778cc719a7e5dde1b2e985..f2ddb202198f0b70b4688438638c2174d1a8d766 100644
--- a/components/session_manager/core/session_manager.h
+++ b/components/session_manager/core/session_manager.h
@@ -8,6 +8,7 @@
 #include <vector>
 
 #include "base/macros.h"
+#include "base/observer_list.h"
 #include "components/session_manager/session_manager_export.h"
 #include "components/session_manager/session_manager_types.h"
 
@@ -15,6 +16,8 @@ class AccountId;
 
 namespace session_manager {
 
+class SessionManagerObserver;
+
 class SESSION_EXPORT SessionManager {
  public:
   SessionManager();
@@ -46,6 +49,9 @@ class SESSION_EXPORT SessionManager {
   // before the session has been started.
   virtual void SessionStarted();
 
+  void AddObserver(SessionManagerObserver* observer);
+  void RemoveObserver(SessionManagerObserver* observer);
+
   SessionState session_state() const { return session_state_; }
   const std::vector<Session>& sessions() const { return sessions_; }
 
@@ -84,6 +90,8 @@ class SESSION_EXPORT SessionManager {
   // Keeps track of user sessions.
   std::vector<Session> sessions_;
 
+  base::ObserverList<SessionManagerObserver> observers_;
+
   DISALLOW_COPY_AND_ASSIGN(SessionManager);
 };