Move enable_webrtc to a buildflag header.

remoting/host/win/BUILD.gn

 # Copyright 2016 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/toolchain/win/midl.gni")
+import("//build/win/message_compiler.gni")
+import("//media/media_options.gni")
 import("//remoting/build/config/remoting_build.gni")
 import("//remoting/host/installer/win/generate_clsids.gni")
-import("//build/toolchain/win/midl.gni")
-import("//build/win/message_compiler.gni")
 
 group("all") {
   testonly = true
 
   deps = [
     ":remoting_console",
     ":remoting_desktop",
     ":remoting_me2me_host",
     ":remoting_native_messaging_host",
     "//remoting/host/security_key:remote_security_key",
@@ skipping 461 matching lines @@
 
   output = "$root_gen_dir/remoting/{{source_name_part}}"
 
   locale_dir = webapp_locale_dir
 
   encoding = "utf-16"
 
   locales = remoting_locales
 }
 # TODO(GYP) More Windows remoting targets from remoting_host_win.gypi