Make GCMDriver::GetInstanceIDHandler protected, to avoid misuse

components/gcm_driver/instance_id/instance_id_driver.h

 // Copyright 2015 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 COMPONENTS_GCM_DRIVER_INSTANCE_ID_INSTANCE_ID_DRIVER_H_
 #define COMPONENTS_GCM_DRIVER_INSTANCE_ID_INSTANCE_ID_DRIVER_H_
 
 #include <map>
 #include <string>
 
@@ skipping 24 matching lines @@
 
   // Removes the InstanceID when it is not longer needed, i.e. the app is being
   // uninstalled.
   void RemoveInstanceID(const std::string& app_id);
 
   // Returns true if the InstanceID for the given application has been created.
   // This is currently only used for testing purpose.
   bool ExistsInstanceID(const std::string& app_id) const;
 
  private:
-  gcm::GCMDriver* gcm_driver_;  // Not owned.
+  // Owned by GCMProfileServiceFactory, which is a dependency of
+  // InstanceIDProfileServiceFactory, which owns this.
+  gcm::GCMDriver* gcm_driver_;
+
   std::map<std::string, scoped_ptr<InstanceID>> instance_id_map_;
 
   DISALLOW_COPY_AND_ASSIGN(InstanceIDDriver);
 };
 
 }  // namespace instance_id
 
 #endif  // COMPONENTS_GCM_DRIVER_INSTANCE_ID_INSTANCE_ID_DRIVER_H_