[Sync] Clear LocalDeviceInfoProvider cache on sync DB destruction.

components/sync_driver/local_device_info_provider.h

 // Copyright 2014 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_SYNC_DRIVER_LOCAL_DEVICE_INFO_PROVIDER_H_
 #define COMPONENTS_SYNC_DRIVER_LOCAL_DEVICE_INFO_PROVIDER_H_
 
 #include <memory>
 #include <string>
 
 #include "base/callback_list.h"
 
 namespace base {
 class TaskRunner;
 }
 
 namespace sync_driver {
 
 class DeviceInfo;
 
 // Interface for providing sync specific information about the
 // local device.
 class LocalDeviceInfoProvider {
  public:
   typedef base::CallbackList<void(void)>::Subscription Subscription;
 
   virtual ~LocalDeviceInfoProvider() {}
 
-  // Returns sync's representation of the local device info;
-  // NULL if the device info is unavailable.
-  // The returned object is fully owned by LocalDeviceInfoProvider (must not
-  // be freed by the caller). It remains valid until LocalDeviceInfoProvider
-  // is destroyed.
+  // Returns sync's representation of the local device info, or nullptr if the
+  // device info is unavailable. The returned object is fully owned by
+  // LocalDeviceInfoProvider; it must not be freed by the caller and should not
+  // be stored.
   virtual const DeviceInfo* GetLocalDeviceInfo() const = 0;
 
   // Constructs a user agent string (ASCII) suitable for use by the syncapi
   // for any HTTP communication. This string is used by the sync backend for
   // classifying client types when calculating statistics.
   virtual std::string GetSyncUserAgent() const = 0;
 
   // Returns a GUID string used for creation of the machine tag for
   // this local session; an empty sting if LocalDeviceInfoProvider hasn't been
   // initialized yet.
   virtual std::string GetLocalSyncCacheGUID() const = 0;
 
   // Starts initializing local device info.
   virtual void Initialize(
       const std::string& cache_guid,
       const std::string& signin_scoped_device_id,
       const scoped_refptr<base::TaskRunner>& blocking_task_runner) = 0;
 
   // Registers a callback to be called when local device info becomes available.
   // The callback will remain registered until the
   // returned Subscription is destroyed, which must occur before the
   // CallbackList is destroyed.
   virtual std::unique_ptr<Subscription> RegisterOnInitializedCallback(
       const base::Closure& callback) = 0;
+
+  // Clears all cached data, returning to an uninitialized state.
+  virtual void Clear() = 0;
 };
 
 }  // namespace sync_driver
 
 #endif  // COMPONENTS_SYNC_DRIVER_LOCAL_DEVICE_INFO_PROVIDER_H_