[ObjC ARC] Converts ios/web/app to ARC

ios/web/app/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.
 
 source_set("app") {
   visibility = [ "//ios/web/public/app" ]
 
   sources = [
     "web_main.mm",
     "web_main_loop.h",
     "web_main_loop.mm",
     "web_main_runner.mm",
   ]
 
+  configs += [ "//build/config/compiler:enable_arc" ]
+
   deps = [
     "//base",
     "//base:i18n",
     "//crypto",
     "//ios/web",
     "//net",
     "//ui/base",
     "//ui/gfx",
     "//ui/gfx/geometry",
   ]
 
   libs = [
     "Foundation.framework",
     "UIKit.framework",
   ]
 }