Convert ios_web_shell to ARC.

ios/web/shell/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/config/ios/rules.gni")
 
 ios_app_bundle("ios_web_shell") {
   info_plist = "Info.plist"
 
   sources = [
     "web_exe_main.mm",
   ]
 
   deps = [
     ":shell",
 
     # All shared libraries must have the sanitizer deps to properly link in
     # asan mode (this target will be empty in other cases).
     "//build/config/sanitizers:deps",
   ]
 
   ldflags = [
     "-Xlinker",
     "-objc_abi_version",
     "-Xlinker",
     "2",
   ]
+
+  configs += [ "//build/config/compiler:enable_arc" ]
 }
 
 bundle_data_xib("main_view_bundle_data") {
   visibility = [ ":shell" ]
   source = "MainView.xib"
 }
 
 bundle_data("shell_bundle_data") {
   visibility = [ ":shell" ]
   sources = [
@@ skipping 39 matching lines @@
     "//net:extras",
     "//ui/base",
   ]
 
   libs = [
     "CoreGraphics.framework",
     "CoreFoundation.framework",
     "Foundation.framework",
     "UIKit.framework",
   ]
+
+  configs += [ "//build/config/compiler:enable_arc" ]
 }