Do not observe MessageLoop destruction from ModelSafeWorker.

components/sync/driver/glue/sync_backend_host.h

 // Copyright 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_SYNC_DRIVER_GLUE_SYNC_BACKEND_HOST_H_
 #define COMPONENTS_SYNC_DRIVER_GLUE_SYNC_BACKEND_HOST_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 99 matching lines @@
       WARN_UNUSED_RESULT = 0;
 
   // Called on |frontend_loop_| to kick off shutdown procedure.  Attempts to cut
   // short any long-lived or blocking sync thread tasks so that the shutdown on
   // sync thread task that we're about to post won't have to wait very long.
   virtual void StopSyncingForShutdown() = 0;
 
   // Called on |frontend_loop_| to kick off shutdown.
   // See the implementation and Core::DoShutdown for details.
   // Must be called *after* StopSyncingForShutdown.
-  // For any reason other than BROWSER_SHUTDOWN, caller should claim sync
-  // thread because:
-  // * during browser shutdown sync thread is not claimed to avoid blocking
-  //   browser shutdown on sync shutdown.
-  // * otherwise sync thread is claimed so that if sync backend is recreated
-  //   later, initialization of new backend is serialized on previous sync
-  //   thread after cleanup of previous backend to avoid old/new backends
-  //   interfere with each other.
+  // Transfers ownership of the sync thread to the caller which may reuse it
+  // with a different SyncBackendHost or join it immediately.
   virtual std::unique_ptr<base::Thread> Shutdown(ShutdownReason reason) = 0;
 
   // Removes all current registrations from the backend on the
   // InvalidationService.
   virtual void UnregisterInvalidationIds() = 0;
 
   // Changes the set of data types that are currently being synced.
   // The ready_task will be run when configuration is done with the
   // set of all types that failed configuration (i.e., if its argument
   // is non-empty, then an error was encountered).
@@ skipping 75 matching lines @@
   // See SyncManager::OnCookieJarChanged.
   virtual void OnCookieJarChanged(bool account_mismatch, bool empty_jar) = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(SyncBackendHost);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_DRIVER_GLUE_SYNC_BACKEND_HOST_H_