Move enable_webrtc to a buildflag header.

chrome/tools/service_discovery_sniffer/BUILD.gn

 # Copyright 2014 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/features.gni")
+import("//media/media_options.gni")
 
 executable("service_discovery_sniffer") {
   testonly = true
   sources = [
     "service_discovery_sniffer.cc",
     "service_discovery_sniffer.h",
   ]
 
   deps = [
     "//base",
     "//base/test:test_support",
     "//build/config/sanitizers:deps",
     "//chrome/browser",
     "//net",
   ]
 
   if (enable_webrtc) {
     deps += [ "//third_party/libjingle/webrtc:libjingle_webrtc" ]
   }
 }