[ObjC ARC] Converts ios/chrome/test:test_support to ARC.

ios/chrome/test/ios_chrome_scoped_testing_chrome_browser_provider.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/test/ios_chrome_scoped_testing_chrome_browser_provider.h"
 
 #include "base/logging.h"
 #import "ios/public/provider/chrome/browser/chrome_browser_provider.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 IOSChromeScopedTestingChromeBrowserProvider::
     IOSChromeScopedTestingChromeBrowserProvider(
         std::unique_ptr<ios::ChromeBrowserProvider> chrome_browser_provider)
     : chrome_browser_provider_(std::move(chrome_browser_provider)),
       original_chrome_browser_provider_(ios::GetChromeBrowserProvider()) {
   ios::SetChromeBrowserProvider(chrome_browser_provider_.get());
 }
 
 IOSChromeScopedTestingChromeBrowserProvider::
     ~IOSChromeScopedTestingChromeBrowserProvider() {
   DCHECK_EQ(chrome_browser_provider_.get(), ios::GetChromeBrowserProvider());
   ios::SetChromeBrowserProvider(original_chrome_browser_provider_);
 }