Modifies BlockCleanupTest so that it can be used from ARC code.

ios/chrome/test/block_cleanup_test.h

 // Copyright (c) 2012 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.
 
 #ifndef IOS_CHROME_TEST_BLOCK_CLEANUP_TEST_H_
 #define IOS_CHROME_TEST_BLOCK_CLEANUP_TEST_H_
 
 #import <Foundation/Foundation.h>
 
 #include "testing/platform_test.h"
 
 @class NSAutoreleasePool;
 
 // Extends PlatformTest to provide a TearDown() method that spins the runloop
 // for a short time.  This allows any blocks created during tests to clean up
 // after themselves.
 class BlockCleanupTest : public PlatformTest {
  protected:
   void SetUp() override;
   void TearDown() override;
 
   void SpinRunLoop(NSTimeInterval cleanup_time);
 
  private:
-  NSAutoreleasePool* block_cleanup_pool_;
+  id block_cleanup_pool_;
 };
 
 #endif  // IOS_CHROME_TEST_BLOCK_CLEANUP_TEST_H_