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

ios/chrome/browser/callback_counter_unittest.mm

Index: ios/chrome/browser/callback_counter_unittest.mm
diff --git a/ios/chrome/browser/callback_counter_unittest.mm b/ios/chrome/browser/callback_counter_unittest.mm
index f257245bd0b8d5f801af96821cd7493922c6d5f9..848ab2afb7bfb83bb8dc2747b1fc1d299c51e5cc 100644
--- a/ios/chrome/browser/callback_counter_unittest.mm
+++ b/ios/chrome/browser/callback_counter_unittest.mm
@@ -10,11 +10,15 @@
 #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;
       }));