[ARC] Converts ios/web:user_agent to ARC.

ios/web/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("//ios/build/config.gni")
 import("//ios/web/js_compile.gni")
 import("//testing/test.gni")
 import("//tools/grit/grit_rule.gni")
 
 source_set("web_arc") {
@@ skipping 307 matching lines @@
 
 source_set("user_agent") {
   deps = [
     "//base",
   ]
 
   sources = [
     "public/user_agent.h",
     "public/user_agent.mm",
   ]
+
+  configs += [ "//build/config/compiler:enable_arc" ]
 }
 
 source_set("earl_grey_test_support") {
   testonly = true
 
   deps = [
     ":test_support",
     ":web",
     "//base",
     "//base/test:test_support",
@@ skipping 273 matching lines @@
 }
 
 grit("resources") {
   source = "ios_web_resources.grd"
   use_qualified_include = true
   outputs = [
     "grit/ios_web_resources.h",
     "ios_web_resources.pak",
   ]
 }