Remove use of deprecated MessageLoop method in ios.

ios/chrome/browser/search_engines/search_engines_util.cc

 // 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.
 
 #include "ios/chrome/browser/search_engines/search_engines_util.h"
 
 #include <stddef.h>
 
-#include "base/message_loop/message_loop.h"
+#include "base/location.h"
+#include "base/single_thread_task_runner.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "components/prefs/pref_service.h"
 #include "components/search_engines/search_engines_pref_names.h"
 #include "components/search_engines/template_url_prepopulate_data.h"
 #include "components/search_engines/template_url_service.h"
 #include "components/search_engines/template_url_service_observer.h"
 
 namespace {
 
 // Id of the google id in template_url_prepopulate_data.cc.
 static const int kGoogleEnginePrepopulatedId = 1;
@@ skipping 48 matching lines @@
     service_->AddObserver(this);
     service_->Load();
   }
 
   ~LoadedObserver() override { service_->RemoveObserver(this); }
 
   void OnTemplateURLServiceChanged() override {
     service_->RemoveObserver(this);
     UpdateSearchEngine(service_);
     // Only delete this class when this callback is finished.
-    base::MessageLoop::current()->DeleteSoon(FROM_HERE, this);
+    base::ThreadTaskRunnerHandle::Get()->DeleteSoon(FROM_HERE, this);
   }
 
  private:
   TemplateURLService* service_;
 };
 
 }  // namespace
 
 namespace search_engines {
 
@@ skipping 17 matching lines @@
   // removed then the engines can be updated again.
   if (!service || service->is_default_search_managed())
     return;
   if (service->loaded())
     UpdateSearchEngine(service);
   else
     new LoadedObserver(service);  // The observer manages its own lifetime.
 }
 
 }  // namespace search_engines