Return unique_ptr from WebClient::CreateWebMainParts()

ios/web/app/web_main_loop.mm

 // Copyright 2014 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/web/app/web_main_loop.h"
 
 #include <stddef.h>
 
 #include <utility>
 
@@ skipping 85 matching lines @@
   DCHECK(!g_current_web_main_loop);
   g_current_web_main_loop = this;
 }
 
 WebMainLoop::~WebMainLoop() {
   DCHECK_EQ(this, g_current_web_main_loop);
   g_current_web_main_loop = nullptr;
 }
 
 void WebMainLoop::Init() {
-  parts_.reset(web::GetWebClient()->CreateWebMainParts());
+  parts_ = web::GetWebClient()->CreateWebMainParts();
 }
 
 void WebMainLoop::EarlyInitialization() {
   if (parts_) {
     parts_->PreEarlyInitialization();
     parts_->PostEarlyInitialization();
   }
 }
 
 void WebMainLoop::MainMessageLoopStart() {
@@ skipping 219 matching lines @@
   main_thread_.reset(
       new WebThreadImpl(WebThread::UI, base::MessageLoop::current()));
 }
 
 int WebMainLoop::WebThreadsStarted() {
   cookie_notification_bridge_.reset(new CookieNotificationBridge);
   return result_code_;
 }
 
 }  // namespace web