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

ios/chrome/browser/BUILD.gn

 # 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.
 
 import("//build/buildflag_header.gni")
 import("//build/config/features.gni")
 import("//build/config/ios/rules.gni")
 import("//rlz/features/features.gni")
 import("//third_party/protobuf/proto_library.gni")
 
@@ skipping 218 matching lines @@
     "//components/variations",
     "//ios/web:test_support",
     "//net",
     "//net:test_support",
     "//testing/gtest",
     "//url",
   ]
 }
 
 source_set("browser_internal") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "callback_counter.h",
     "callback_counter.mm",
   ]
   deps = [
     "//base",
   ]
 }