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

ios/chrome/browser/callback_counter_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.
 
 #include "ios/chrome/browser/callback_counter.h"
 
 #include "base/bind.h"
 #include "base/mac/bind_objc_block.h"
 #include "base/memory/ref_counted.h"
 #import "base/test/ios/wait_util.h"
 #include "testing/platform_test.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 // Tests that CallbackCounter works with 2 callbacks.
 TEST(CallbackCounterTest, Basic) {
   __block BOOL block_was_called = NO;
   scoped_refptr<CallbackCounter> callback_counter =
-      new CallbackCounter(base::BindBlock(^{
+      new CallbackCounter(base::BindBlockArc(^{
         block_was_called = YES;
       }));
 
   // Enqueue the first callback.
   callback_counter->IncrementCount();
   dispatch_async(dispatch_get_main_queue(), ^{
     callback_counter->DecrementCount();
   });
 
   // Enqueue the second callback.
   callback_counter->IncrementCount();
   dispatch_async(dispatch_get_main_queue(), ^{
     callback_counter->DecrementCount();
   });
 
   base::test::ios::WaitUntilCondition(^bool() {
     return block_was_called;
   });
 }