[ObjC ARC] Converts ios/chrome/browser:unit_tests to ARC.

ios/chrome/browser/ios_chrome_io_thread_unittest.mm

 // Copyright 2016 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/ios_chrome_io_thread.h"
 
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "components/prefs/pref_registry_simple.h"
 #include "components/prefs/pref_service.h"
 #include "components/prefs/pref_service_factory.h"
 #include "components/prefs/testing_pref_store.h"
 #include "components/proxy_config/pref_proxy_config_tracker_impl.h"
 #include "components/ssl_config/ssl_config_service_manager.h"
 #include "ios/web/public/test/test_web_thread.h"
 #include "net/test/url_request/url_request_failed_job.h"
 #include "net/url_request/url_fetcher.h"
 #include "net/url_request/url_fetcher_delegate.h"
 #include "net/url_request/url_request_filter.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/gtest_mac.h"
 #include "testing/platform_test.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 
 // A delegate interface for users of URLFetcher.
 class TestURLFetcherDelegate : public net::URLFetcherDelegate {
  public:
   TestURLFetcherDelegate() {}
   ~TestURLFetcherDelegate() override {}
 
   // net::URLFetcherDelegate implementation.
   void OnURLFetchComplete(const net::URLFetcher* source) override {}
@@ skipping 44 matching lines @@
   std::unique_ptr<net::URLFetcher> fetcher = net::URLFetcher::Create(
       net::URLRequestFailedJob::GetMockHttpUrl(net::ERR_IO_PENDING),
       net::URLFetcher::GET, &fetcher_delegate);
   fetcher->SetRequestContext(
       ios_chrome_io_thread->system_url_request_context_getter());
   fetcher->Start();
   base::RunLoop().RunUntilIdle();
   // Verify that there is no AssertNoUrlRequests triggered during CleanUp.
   web_thread_delegate->CleanUp();
 }