Migrate upgrade notifications to observer

chrome/browser/upgrade_observer.h

 // Copyright 2017 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.
 
 #ifndef CHROME_BROWSER_UPGRADE_OBSERVER_H_
 #define CHROME_BROWSER_UPGRADE_OBSERVER_H_
 
 #include "base/macros.h"
 
 class UpgradeObserver {
  public:
   // Triggered when a software update is available, but downloading requires
   // user's agreement as current connection is cellular.
-  virtual void OnUpdateOverCellularAvailable() = 0;
+  virtual void OnUpdateOverCellularAvailable() {}
+
+  // Triggered when Chrome believes an update has been installed and available
+  // for long enough with the user shutting down to let it take effect. See
+  // upgrade_detector.cc for details on how long it waits. No details are
+  // expected.
+  virtual void OnUpgradeRecommended() {}
+
+  // Triggered when a critical update has been installed. No details are
+  // expected.
+  virtual void OnCriticalUpgradeInstalled() {}
+
+  // Triggered when the current install is outdated. No details are expected.
+  virtual void OnOutdatedInstall() {}
+
+  // Triggered when the current install is outdated and auto-update (AU) is
+  // disabled. No details are expected.
+  virtual void OnOutdatedInstallNoAutoUpdate() {}
 
  protected:
   virtual ~UpgradeObserver() {}
 };
 
 #endif  // CHROME_BROWSER_UPGRADE_OBSERVER_H_