[Sync] Move sync thread ownership to ProfileSyncService.

components/browser_sync/profile_sync_service.h

 // Copyright (c) 2012 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_BROWSER_SYNC_PROFILE_SYNC_SERVICE_H_
 #define COMPONENTS_BROWSER_SYNC_PROFILE_SYNC_SERVICE_H_
 
 #include <memory>
 #include <set>
 #include <string>
@@ skipping 903 matching lines @@
 
   syncer::DataTypeManager::ConfigureStatus configure_status_;
 
   // The set of currently enabled sync experiments.
   syncer::Experiments current_experiments_;
 
   // Sync's internal debug info listener. Used to record datatype configuration
   // and association information.
   syncer::WeakHandle<syncer::DataTypeDebugInfoListener> debug_info_listener_;
 
-  // A thread where all the sync operations happen.
-  // OWNERSHIP Notes:
-  //     * Created when backend starts for the first time.
-  //     * If sync is disabled, PSS claims ownership from backend.
-  //     * If sync is reenabled, PSS passes ownership to new backend.
+  // The thread where all the sync operations happen. This thread is kept alive
+  // until browser shutdown and reused if sync is turned off and on again. It is
+  // joined during the shutdown process, but there is an abort mechanism in
+  // place to prevent slow HTTP requests from blocking browser shutdown.
   std::unique_ptr<base::Thread> sync_thread_;
 
   // ProfileSyncService uses this service to get access tokens.
   ProfileOAuth2TokenService* const oauth2_token_service_;
 
   // ProfileSyncService needs to remember access token in order to invalidate it
   // with OAuth2TokenService.
   std::string access_token_;
 
   // ProfileSyncService needs to hold reference to access_token_request_ for
@@ skipping 69 matching lines @@
   base::WeakPtrFactory<ProfileSyncService> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ProfileSyncService);
 };
 
 bool ShouldShowActionOnUI(const syncer::SyncProtocolError& error);
 
 }  // namespace browser_sync
 
 #endif  // COMPONENTS_BROWSER_SYNC_PROFILE_SYNC_SERVICE_H_