Make GCMDriver::GetInstanceIDHandler protected, to avoid misuse

components/gcm_driver/instance_id/instance_id.cc

 // 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.
 
 #include "components/gcm_driver/instance_id/instance_id.h"
 
 namespace instance_id {
 
-InstanceID::InstanceID(const std::string& app_id)
-    : app_id_(app_id) {
+InstanceID::InstanceID(const std::string& app_id,
+                       gcm::InstanceIDHandler* handler)
+    : handler_(handler), app_id_(app_id) {
+  DCHECK(handler_);
 }
 
 InstanceID::~InstanceID() {
 }
 
 void InstanceID::SetTokenRefreshCallback(const TokenRefreshCallback& callback) {
   token_refresh_callback_ = callback;
 }
 
 void InstanceID::NotifyTokenRefresh(bool update_id) {
   if (!token_refresh_callback_.is_null())
     token_refresh_callback_.Run(app_id_, update_id);
 }
 
 }  // namespace instance_id