Do not observe MessageLoop destruction from ModelSafeWorker.

components/sync/driver/sync_client.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_SYNC_DRIVER_SYNC_CLIENT_H_
 #define COMPONENTS_SYNC_DRIVER_SYNC_CLIENT_H_
 
 #include "base/callback_forward.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
@@ skipping 86 matching lines @@
   // Returns a weak pointer to the ModelTypeSyncBridge specified by |type|. Weak
   // pointer may be unset if service is already destroyed.
   // Note: Should only be dereferenced from the model type thread.
   virtual base::WeakPtr<ModelTypeSyncBridge> GetSyncBridgeForModelType(
       ModelType type) = 0;
 
   // Creates and returns a new ModelSafeWorker for the group, or null if one
   // cannot be created.
   // TODO(maxbogue): Move this inside SyncApiComponentFactory.
   virtual scoped_refptr<ModelSafeWorker> CreateModelWorkerForGroup(
-      ModelSafeGroup group,
-      WorkerLoopDestructionObserver* observer) = 0;
+      ModelSafeGroup group) = 0;
 
   // Returns the current SyncApiComponentFactory instance.
   virtual SyncApiComponentFactory* GetSyncApiComponentFactory() = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(SyncClient);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_DRIVER_SYNC_CLIENT_H_