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

ios/third_party/ochamcrest/BUILD.gn

 # Copyright 2016 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.
 
 # OCHamcrest sources contains import rules using relative file names
 # ("HCAssertThat.h") and other using paths in an installed framework
 # (<OCHamcrest/HCMatcher.h>). In order to build, copy all the sources
 # to <(SHARED_INTERMEDIATE_DIR)/ios/third_party/ochmacrest/OCHamcrest
 # so that both type of import work (another option considered was to
 # build forwarding headers but this required duplicating the list of
@@ skipping 157 matching lines @@
     "src/Source/Library/Text/HCStringStartsWith.m",
     "src/Source/Library/Text/HCSubstringMatcher.h",
     "src/Source/Library/Text/HCSubstringMatcher.m",
     "src/Source/OCHamcrest.h",
   ]
   outputs = [
     "$root_gen_dir/ios/third_party/ochamcrest/OCHamcrest/{{source_file_part}}",
   ]
 }
 
-config("ochamcrest_config") {
+config("config") {
   visibility = [ ":ochamcrest" ]
   libs = [ "XCTest.framework" ]
   include_dirs = [ "$root_gen_dir/ios/third_party/ochamcrest" ]
 }
 
-config("ochamcrest_private_config") {
-  visibility = [ ":ochamcrest" ]
-  cflags_objc = [ "-fobjc-arc" ]
-}
-
 source_set("ochamcrest") {
   testonly = true
   configs += [
-    ":ochamcrest_config",
-    ":ochamcrest_private_config",
+    ":config",
+    "//build/config/compiler:enable_arc",
     "//build/config/compiler:no_chromium_code",
   ]
-  public_configs = [ ":ochamcrest_config" ]
+  public_configs = [ ":config" ]
   public_deps = [
     ":ochamcrest_copy_files",
   ]
   sources = get_target_outputs(":ochamcrest_copy_files")
 }