Move enable_webrtc to a buildflag header.

remoting/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("//media/media_options.gni")
 import("//remoting/build/config/remoting_build.gni")
 
 group("remoting_all") {
   testonly = true
 
   deps = [
     ":remoting_unittests",
     "//remoting/test:chromoting_test_driver",
     "//remoting/webapp:browser_test_resources",
     "//remoting/webapp:unit_tests",
@@ skipping 198 matching lines @@
     if (is_win) {
       defines += [ "_ALT_NO_EXCEPTIONS" ]
 
       libs += [
         "rpcrt4.lib",
         "wtsapi32.lib",
       ]
     }
   }
 }