Don't check is_ios in chrome/, it's always false there.

chrome/common/features.gni

 # 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/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/features.gni")
 import("//device/vr/features.gni")
 import("//extensions/features/features.gni")
 import("//media/media_options.gni")
 import("//net/features.gni")
 import("//ppapi/features/features.gni")
 import("//printing/features/features.gni")
 import("//ui/base/ui_features.gni")
 
+assert(!is_ios, "Chromium/iOS shouldn't use anything in //chrome")
+
 # Please keep features in alphabetical order.
 declare_args() {
   enable_app_list = is_chromeos
 
   # Enables support for background apps.
-  enable_background = !is_ios && !is_android && !is_chromecast
+  enable_background = !is_android && !is_chromecast
 
   # Enable the printing system dialog for platforms that support printing
   # and have a system dialog.
   enable_basic_print_dialog = enable_basic_printing && !is_chromeos
 
-  enable_captive_portal_detection = !is_android && !is_ios && !is_chromecast
+  enable_captive_portal_detection = !is_android && !is_chromecast
 
   # Google Now is disabled to prepare for its removal.
   # http://crbug.com/539674
   enable_google_now = false
 
   # Hangout services is an extension that adds extra features to Hangouts.
   # It is enableable separately to facilitate testing.
   enable_hangout_services_extension = is_chrome_branded
 
   # 'Ok Google' hotwording is disabled by default. Set to true to enable. (This
   # will download a closed-source NaCl module at startup.) Chrome-branded
   # ChromeOS builds have this enabled by default.
   enable_hotwording = is_chrome_branded && is_chromeos
 
   enable_one_click_signin =
       is_win || is_mac || (is_linux && !is_chromeos && !is_chromecast)
 
   # Set to true to bundle all the mash related mojo services into chrome.
   # Specify --mash to chrome to have chrome start the mash environment.
   enable_package_mash_services = is_chromeos
 
   enable_service_discovery = enable_mdns || is_mac
 
   # Enables use of the session service, which is enabled by default.
   # Android stores them separately on the Java side.
-  enable_session_service = !is_android && !is_ios && !is_chromecast
+  enable_session_service = !is_android && !is_chromecast
 
-  enable_supervised_users = !is_ios && !is_chromecast
+  enable_supervised_users = !is_chromecast
 
   # Enables vr shell UI development on local or remote page.
   enable_vr_shell_ui_dev = false
 
   # Indicates if Wayland display server support is enabled.
   enable_wayland_server = is_chromeos
 
   # Enable native notifications via XPC services (mac only).
   # TODO(miguelg): remove this.
   enable_xpc_notifications = true
@@ skipping 25 matching lines @@
   "enable_plugins=$enable_plugins",
   "enable_print_preview=$enable_print_preview",
   "enable_printing=$_enable_printing",
   "enable_service_discovery=$enable_service_discovery",
   "enable_webrtc=$enable_webrtc",
   "enable_webvr=$enable_webvr",
   "mac_views_browser=$mac_views_browser",
   "safe_browsing_mode=$safe_browsing_mode",
   "use_vulcanize=$use_vulcanize",
 ]