Add SessionStateObserver with ActiveUserChanged()

chrome/browser/chromeos/login/user_manager.h

Index: chrome/browser/chromeos/login/user_manager.h
diff --git a/chrome/browser/chromeos/login/user_manager.h b/chrome/browser/chromeos/login/user_manager.h
index 059d888094aef59defc81dde92422b2c9c12e2e3..9c1b84c525bab259fbd3a6c740add4b3b86d08e6 100644
--- a/chrome/browser/chromeos/login/user_manager.h
+++ b/chrome/browser/chromeos/login/user_manager.h
@@ -49,12 +49,15 @@ class UserManager {
   // TODO(nkostylev): Merge with session state refactoring CL.
   class UserSessionStateObserver {
    public:
+    // Called when active user has changed.
+    virtual void ActiveUserChanged(const User* active_user) {}
+
     // Called right before notifying on user change so that those who rely
     // on user_id hash would be accessing up-to-date value.
-    virtual void ActiveUserHashChanged(const std::string& hash) = 0;
+    virtual void ActiveUserHashChanged(const std::string& hash) {}
 
     // Called when UserManager finishes restoring user sessions after crash.
-    virtual void PendingUserSessionsRestoreFinished() = 0;
+    virtual void PendingUserSessionsRestoreFinished() {}
 
    protected:
     virtual ~UserSessionStateObserver();