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

ios/chrome/browser/callback_counter.mm

Index: ios/chrome/browser/callback_counter.mm
diff --git a/ios/chrome/browser/callback_counter.mm b/ios/chrome/browser/callback_counter.mm
index ffe4642a4a74c757ee034b591249d46afba7b1a6..73b1aa26ad3775aad73c3cdff725a820d56f8c47 100644
--- a/ios/chrome/browser/callback_counter.mm
+++ b/ios/chrome/browser/callback_counter.mm
@@ -4,6 +4,10 @@
 
 #include "ios/chrome/browser/callback_counter.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 CallbackCounter::CallbackCounter(const FinalCallback& final_callback)
     : callback_count_(0U), final_callback_(final_callback) {
   DCHECK(!final_callback.is_null());