[ObjC ARC] Converts ios/chrome/browser:browser 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("//third_party/protobuf/proto_library.gni")
 
 declare_args() {
   google_test_gaia_client_id = ""
   google_test_gaia_client_secret = ""
   google_staging_api_url = ""
   google_staging_lso_url = ""
   google_test_api_url = ""
   google_test_lso_url = ""
   google_test_oauth_client_id = ""
   google_test_oauth_client_secret = ""
   google_test_oauth_url = ""
   google_test_sync_url = ""
 }
 
 source_set("browser") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "about_flags.h",
     "about_flags.mm",
     "app_startup_parameters.h",
     "app_startup_parameters.mm",
     "application_context.cc",
     "application_context.h",
     "arch_util.cc",
     "arch_util.h",
     "browser_about_rewriter.cc",
@@ skipping 199 matching lines @@
     "//components/proxy_config",
     "//components/ssl_config",
     "//components/variations",
     "//ios/web:test_support",
     "//net",
     "//net:test_support",
     "//testing/gtest",
     "//url",
   ]
 }