Move enable_webrtc to a buildflag header.

media/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/buildflag_header.gni")
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/features.gni")
 import("//build/config/linux/pkg_config.gni")
 import("//build/config/ui.gni")
 import("//media/media_options.gni")
 import("//ppapi/features/features.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 import("//testing/test.gni")
 import("//third_party/ffmpeg/ffmpeg_options.gni")
 
 buildflag_header("media_features") {
   header = "media_features.h"
 
   flags = [
     "ENABLE_AC3_EAC3_AUDIO_DEMUXING=$enable_ac3_eac3_audio_demuxing",
     "ENABLE_HEVC_DEMUXING=$enable_hevc_demuxing",
     "ENABLE_MSE_MPEG2TS_STREAM_PARSER=$enable_mse_mpeg2ts_stream_parser",
     "ENABLE_MEDIA_REMOTING=$enable_media_remoting",
+    "ENABLE_WEBRTC=$enable_webrtc",
   ]
 }
 
 if (proprietary_codecs && media_use_ffmpeg) {
   assert(
       ffmpeg_branding != "Chromium",
       "proprietary codecs and ffmpeg_branding set to Chromium are incompatible")
 }
 
 # Common configuration for targets in the media directory.
@@ skipping 894 matching lines @@
   fuzzer_test("media_mp4_avcc_parser_fuzzer") {
     sources = [
       "formats/mp4/mp4_avcc_parser_fuzzer.cc",
     ]
     deps = [
       ":media",
       "//base",
     ]
   }
 }