Do not observe MessageLoop destruction from ModelSafeWorker.

ios/chrome/browser/sync/ios_chrome_sync_client.mm

 // 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.
 
 #include "ios/chrome/browser/sync/ios_chrome_sync_client.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/command_line.h"
@@ skipping 324 matching lines @@
                  browser_state_)
           ->GetDeviceInfoSyncBridge()
           ->AsWeakPtr();
     default:
       NOTREACHED();
       return base::WeakPtr<syncer::ModelTypeSyncBridge>();
   }
 }
 
 scoped_refptr<syncer::ModelSafeWorker>
-IOSChromeSyncClient::CreateModelWorkerForGroup(
-    syncer::ModelSafeGroup group,
-    syncer::WorkerLoopDestructionObserver* observer) {
+IOSChromeSyncClient::CreateModelWorkerForGroup(syncer::ModelSafeGroup group) {
   DCHECK_CURRENTLY_ON(web::WebThread::UI);
   switch (group) {
     case syncer::GROUP_DB:
       return new syncer::BrowserThreadModelWorker(
           web::WebThread::GetTaskRunnerForThread(web::WebThread::DB),
-          syncer::GROUP_DB, observer);
+          syncer::GROUP_DB);
     case syncer::GROUP_FILE:
       return new syncer::BrowserThreadModelWorker(
           web::WebThread::GetTaskRunnerForThread(web::WebThread::FILE),
-          syncer::GROUP_FILE, observer);
+          syncer::GROUP_FILE);
     case syncer::GROUP_UI:
       return new syncer::UIModelWorker(
-          web::WebThread::GetTaskRunnerForThread(web::WebThread::UI), observer);
+          web::WebThread::GetTaskRunnerForThread(web::WebThread::UI));
     case syncer::GROUP_PASSIVE:
-      return new syncer::PassiveModelWorker(observer);
+      return new syncer::PassiveModelWorker();
     case syncer::GROUP_HISTORY: {
       history::HistoryService* history_service = GetHistoryService();
       if (!history_service)
         return nullptr;
       return new browser_sync::HistoryModelWorker(
           history_service->AsWeakPtr(),
-          web::WebThread::GetTaskRunnerForThread(web::WebThread::UI), observer);
+          web::WebThread::GetTaskRunnerForThread(web::WebThread::UI));
     }
     case syncer::GROUP_PASSWORD: {
       if (!password_store_)
         return nullptr;
-      return new browser_sync::PasswordModelWorker(password_store_, observer);
+      return new browser_sync::PasswordModelWorker(password_store_);
     }
     default:
       return nullptr;
   }
 }
 
 syncer::SyncApiComponentFactory*
 IOSChromeSyncClient::GetSyncApiComponentFactory() {
   return component_factory_.get();
 }
@@ skipping 16 matching lines @@
         IOSChromeProfileSyncServiceFactory::GetForBrowserState(browser_state);
     if (profile_sync_service != nullptr) {
       const syncer::DeviceInfoTracker* tracker =
           profile_sync_service->GetDeviceInfoTracker();
       if (tracker != nullptr) {
         trackers->push_back(tracker);
       }
     }
   }
 }