Implement InstanceID API functions.

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>
 
 #include "base/macros.h"
 #include "base/stl_util.h"
 
 namespace gcm {
 class GCMDriver;
 }  // namespace gcm
 
 namespace instance_id {
 
 class InstanceID;
 
 // Bridge between Instance ID users in Chrome and the platform-specific
 // implementation.
 class InstanceIDDriver {
  public:
+  // Returns whether InstanceID is enabled.
+  static bool IsInstanceIDEnabled();
+
   explicit InstanceIDDriver(gcm::GCMDriver* gcm_driver);
   virtual ~InstanceIDDriver();
 
   // Returns the InstanceID that provides the Instance ID service for the given
   // application. The lifetime of InstanceID will be managed by this class.
   InstanceID* GetInstanceID(const std::string& app_id);
 
  private:
   gcm::GCMDriver* gcm_driver_;  // Not owned.
   typedef std::map<std::string, InstanceID*> InstanceIDMap;
   InstanceIDMap instance_id_map_;
   STLValueDeleter<InstanceIDMap> instance_id_map_deleter_;
 
   DISALLOW_COPY_AND_ASSIGN(InstanceIDDriver);
 };
 
 }  // namespace instance_id
 
 #endif  // COMPONENTS_GCM_DRIVER_INSTANCE_ID_INSTANCE_ID_DRIVER_H_