Add config to enable ARC on iOS/OS X to //build/config.

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 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/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 
@@ skipping 1317 matching lines @@
   if (symbol_level == 0) {
     configs = [ ":no_symbols" ]
   } else if (symbol_level == 1) {
     configs = [ ":minimal_symbols" ]
   } else if (symbol_level == 2) {
     configs = [ ":symbols" ]
   } else {
     assert(false)
   }
 }
+
+if (is_ios || is_mac) {
+  # On Mac and iOS, this enables support for ARC (automatic ref-counting).
+  # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
+  config("enable_arc") {
+    common_flags = [ "-fobjc-arc" ]
+    cflags_objc = common_flags
+    cflags_objcc = common_flags
+  }
+}