Expose audio codec factories in libwebrtc.

This explicitly exposes the following functions,
without relying on (wrong) transitive dependency.
* CreateBuiltinAudioDecoderFactory()
* CreateBuiltinAudioEncoderFactory()

Manual check:
% gn gen out/test && ninja -C out/test webrtc
% nm -C out/test/obj/libwebrtc.a | grep CreateBuiltinAudio
0000000000000000 T webrtc::CreateBuiltinAudioDecoderFactory()
0000000000000000 T webrtc::CreateBuiltinAudioEncoderFactory()

Bug: webrtc:9528
Change-Id: I0d2b0a8294dc90b553a7dff9045ea6e4f0d0d416
Reviewed-on: https://webrtc-review.googlesource.com/89062
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Yves Gerey <yvesg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24042}

BUILD.gn

@@ -350,6 +350,17 @@ if (!build_with_chromium) {
       "video",
     ]
 
+    # Additional factory functions to be exposed.
+    # Rational: These factories are small enough (89 KiB / 32+ MiB)
+    #           to be unconditionaly included for user convenience.
+    # That begin said:
+    # TODO(yvesg) Consider making all non-core APIs optional, so that users
+    #             can build a customized library tailored to their needs.
+    deps += [
+      "api/audio_codecs:builtin_audio_decoder_factory",
+      "api/audio_codecs:builtin_audio_encoder_factory",
+    ]
+
     if (build_with_mozilla) {
       deps += [
         "api/video:video_frame",