Reimplement the builtin audio codec factories using the new stuff in api/

webrtc/api/audio_codecs/opus/BUILD.gn

 # Copyright (c) 2017 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("../../../webrtc.gni")
 if (is_android) {
@@ skipping 18 matching lines @@
 }
 
 rtc_source_set("audio_encoder_opus") {
   sources = [
     "audio_encoder_opus.h",
   ]
   deps = [
     ":audio_encoder_opus_config",
     "..:audio_codecs_api",
     "../../../modules/audio_coding:webrtc_opus",
-    "../../../rtc_base:protobuf_utils",  # TODO(kwiberg): Why is this needed?
     "../../../rtc_base:rtc_base_approved",
   ]
+  public_deps = [
+    # TODO(kwiberg): Remove this public_dep when bug 7847 has been fixed.
+    "../../../rtc_base:protobuf_utils",
+  ]
 }
 
 rtc_static_library("audio_decoder_opus") {
   sources = [
     "audio_decoder_opus.cc",
     "audio_decoder_opus.h",
   ]
   deps = [
     "..:audio_codecs_api",
     "../../..:webrtc_common",
     "../../../modules/audio_coding:webrtc_opus",
     "../../../rtc_base:rtc_base_approved",
   ]
 }