[ObjC ARC] Converts ios/web/shell/test to ARC.

ios/web/shell/test/earl_grey/shell_base_test_case.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.
 
 #import "ios/web/shell/test/earl_grey/shell_base_test_case.h"
 
 #import <EarlGrey/EarlGrey.h>
 
 #import "ios/web/public/test/http_server.h"
 #import "ios/web/shell/test/earl_grey/shell_matchers.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 using web::test::HttpServer;
 using web::webViewContainingText;
 
 @implementation ShellBaseTestCase
 
 // Set up called once for the class.
 + (void)setUp {
   [super setUp];
   [[EarlGrey selectElementWithMatcher:webViewContainingText("Chromium")]
       assertWithMatcher:grey_notNil()];
   HttpServer::GetSharedInstance().StartOrDie();
 }
 
 // Tear down called once for the class.
 + (void)tearDown {
   HttpServer::GetSharedInstance().Stop();
   [super tearDown];
 }
 
 // Tear down called after each test.
 - (void)tearDown {
   HttpServer::GetSharedInstance().RemoveAllResponseProviders();
   [super tearDown];
 }
 
 @end