[ObjC ARC] Converts ios/web/public/test:test to ARC.

ios/web/public/test/scoped_testing_web_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/web/public/test/scoped_testing_web_client.h"
 
 #include "base/logging.h"
 #import "ios/web/public/web_client.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 ScopedTestingWebClient::ScopedTestingWebClient(
     std::unique_ptr<WebClient> web_client)
     : web_client_(std::move(web_client)), original_web_client_(GetWebClient()) {
   SetWebClient(web_client_.get());
 }
 
 ScopedTestingWebClient::~ScopedTestingWebClient() {
   DCHECK_EQ(GetWebClient(), web_client_.get());
   SetWebClient(original_web_client_);
 }
 
 WebClient* ScopedTestingWebClient::Get() {
   return web_client_.get();
 }
 
 }  // namespace web