[ObjC ARC] Converts ios/web:ios_web_webui_unittests to ARC.

ios/web/webui/url_fetcher_block_adapter_unittest.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.
 
 #import "ios/web/webui/url_fetcher_block_adapter.h"
 
 #include <string>
 
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/run_loop.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/test/scoped_task_environment.h"
 #include "net/url_request/test_url_fetcher_factory.h"
 #include "net/url_request/url_request_status.h"
 #include "net/url_request/url_request_test_util.h"
 #import "testing/gtest_mac.h"
 #include "testing/platform_test.h"
 #include "url/gurl.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 // Test fixture for URLFetcherBlockAdapter.
 class URLFetcherBlockAdapterTest : public PlatformTest {
  protected:
   URLFetcherBlockAdapterTest()
       : scoped_task_environment_(
             base::test::ScopedTaskEnvironment::MainThreadType::UI) {}
 
   // Required for base::MessageLoop::current().
@@ skipping 36 matching lines @@
   std::string response;
   EXPECT_TRUE(ReadFileToString(favicon_path, &response));
   net::FakeURLFetcher fake_fetcher(test_url, &web_ui_fetcher, response,
                                    net::HTTP_OK,
                                    net::URLRequestStatus::SUCCESS);
   fake_fetcher.Start();
   base::RunLoop().RunUntilIdle();
 }
 
 }  // namespace web