Fix Gn untracked headers in webrtc/media

webrtc/media/BUILD.gn

 # Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("//build/config/linux/pkg_config.gni")
 import("../webrtc.gni")
@@ skipping 122 matching lines @@
 
 rtc_static_library("rtc_audio_video") {
   defines = []
   libs = []
   deps = []
   sources = [
     "engine/adm_helpers.cc",
     "engine/adm_helpers.h",
     "engine/apm_helpers.cc",
     "engine/apm_helpers.h",
+    "engine/constants.h",
     "engine/internaldecoderfactory.cc",
     "engine/internaldecoderfactory.h",
     "engine/internalencoderfactory.cc",
     "engine/internalencoderfactory.h",
     "engine/nullwebrtcvideoengine.h",
     "engine/payload_type_mapper.cc",
     "engine/payload_type_mapper.h",
     "engine/scopedvideodecoder.cc",
     "engine/scopedvideodecoder.h",
     "engine/scopedvideoencoder.cc",
@@ skipping 380 matching lines @@
       "../rtc_base:rtc_base_approved",
       "../rtc_base:rtc_base_tests_main",
       "../rtc_base:rtc_base_tests_utils",
       "../system_wrappers:metrics_default",
       "../test:audio_codec_mocks",
       "../test:test_support",
       "../voice_engine:voice_engine",
     ]
   }
 }