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

ios/third_party/gcdwebserver/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.
 
-config("public_config") {
+config("config") {
+  visibility = [ ":gcdwebserver" ]
   libs = [
     "CFNetwork.framework",
     "MobileCoreServices.framework",
     "z",
   ]
 
   include_dirs = [
     "src/GCDWebServer/Core",
     "src/GCDWebServer/Requests",
     "src/GCDWebServer/Responses",
   ]
 }
 
-config("internal_config") {
+config("private_config") {
   visibility = [ ":gcdwebserver" ]
 
   # TODO(crbug.com/569158): Suppresses warnings that are treated as errors
   # when minimum iOS version support is increased to iOS 9 and up.
   # This should be removed once all deprecation violations have been fixed.

[Nico, 2016/03/18 21:52:07]

Somewhat drive-by: FWIW, I feel pretty strongly that this shouldn't have been
added in the first place; see also our previous discussion of this on
https://codereview.chromium.org/1180583002/

Please fix this very soon.

[sdefresne, 2016/03/19 13:34:27]

I agree. pkl@ is owning the removal of those temporary overrides.

   cflags = [ "-Wno-deprecated-declarations" ]
-  cflags_objc = [ "-fobjc-arc" ]
 }
 
 source_set("gcdwebserver") {
   testonly = true
 
   sources = [
     "src/GCDWebServer/Core/GCDWebServer.h",
     "src/GCDWebServer/Core/GCDWebServer.m",
     "src/GCDWebServer/Core/GCDWebServerConnection.h",
     "src/GCDWebServer/Core/GCDWebServerConnection.m",
@@ skipping 16 matching lines @@
     "src/GCDWebServer/Responses/GCDWebServerDataResponse.h",
     "src/GCDWebServer/Responses/GCDWebServerDataResponse.m",
     "src/GCDWebServer/Responses/GCDWebServerErrorResponse.h",
     "src/GCDWebServer/Responses/GCDWebServerErrorResponse.m",
     "src/GCDWebServer/Responses/GCDWebServerFileResponse.h",
     "src/GCDWebServer/Responses/GCDWebServerFileResponse.m",
     "src/GCDWebServer/Responses/GCDWebServerStreamedResponse.h",
     "src/GCDWebServer/Responses/GCDWebServerStreamedResponse.m",
   ]
 
-  public_configs = [ ":public_config" ]
-  configs += [ ":internal_config" ]
+  public_configs = [ ":config" ]
+  configs += [
+    ":private_config",
+    "//build/config/compiler:enable_arc",
+  ]
 }