Move enable_remoting build flag to remoting dir.

remoting/remoting_enable.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.
 
-# Keep in sync with remoting/remoting_enable.gypi.
+import("//build/config/ui.gni")
 
-import("//build/config/ui.gni")
+declare_args() {
+  enable_remoting = !is_ios && !is_chromecast
+}
 
 enable_remoting_host = false
 enable_me2me_host = false
 
 if (is_win || (is_linux && !is_chromeos && use_x11) || is_mac) {
   enable_remoting_host = true
   enable_me2me_host = true
 }
 
 if (is_linux && is_chromeos) {
   enable_remoting_host = true
 }