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

ios/chrome/app/application_delegate/background_activity_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.
 
 #import "ios/chrome/app/application_delegate/background_activity.h"
 
 #import "ios/chrome/app/application_delegate/browser_launcher.h"
 #import "ios/chrome/app/application_delegate/metrics_mediator.h"
 #import "ios/chrome/browser/crash_report/crash_report_background_uploader.h"
 #import "ios/chrome/browser/metrics/previous_session_info.h"
 #import "ios/chrome/browser/metrics/previous_session_info_private.h"
 #import "ios/chrome/test/base/scoped_block_swizzler.h"
 #include "testing/platform_test.h"
 #import "third_party/ocmock/OCMock/OCMock.h"
 #import "third_party/ocmock/gtest_support.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 // Verifies that -application:performFetchWithCompletionHandler: calls the
 // browser launcher in background state and uploads the report.
 TEST(BackgroundActivityTest, performFetchWithCompletionHandler) {
   // Setup.
   [[PreviousSessionInfo sharedInstance] setIsFirstSessionAfterUpgrade:NO];
 
   // MetricsMediator mock.
   id metrics_mediator_mock =
       [OCMockObject mockForClass:[MetricsMediator class]];
   [[[metrics_mediator_mock stub] andReturnValue:@YES] areMetricsEnabled];
@@ skipping 37 matching lines @@
 
   // Test.
   [BackgroundActivity handleEventsForBackgroundURLSession:nil
                                         completionHandler:^{
                                         }
                                           browserLauncher:browser_launcher];
 
   // Check.
   EXPECT_OCMOCK_VERIFY(browser_launcher);
 }