Injectable audio encoders: BuiltinAudioEncoderFactory

This CL contains all the changes made to audio_coding while making
audio encoders injectable. Apart from some small changes to
webrtcvoiceengine, nothing here is hooked up to the outside
world. Those changes will be added to a follow-up CL.

BUG=webrtc:5806

Review-Url: https://codereview.webrtc.org/2695243005
Cr-Commit-Position: refs/heads/master@{#17569}

webrtc/api/peerconnectioninterface.h

@@ -91,6 +91,9 @@
 #include "webrtc/base/sslstreamadapter.h"
 #include "webrtc/media/base/mediachannel.h"
 #include "webrtc/media/base/videocapturer.h"
+#include "webrtc/modules/audio_coding/codecs/audio_encoder_factory.h"
+// TODO(ossu): Remove this once downstream projects have been updated.
+#include "webrtc/modules/audio_coding/codecs/builtin_audio_encoder_factory.h"
 #include "webrtc/p2p/base/portallocator.h"
 
 namespace rtc {
@@ -986,14 +989,6 @@ class PeerConnectionFactoryInterface : public rtc::RefCountInterface {
   ~PeerConnectionFactoryInterface() {} // NOLINT
 };
 
-// TODO(ossu): Remove these and define a real builtin audio encoder factory
-// instead.
-class AudioEncoderFactory : public rtc::RefCountInterface {};
-inline rtc::scoped_refptr<AudioEncoderFactory>
-CreateBuiltinAudioEncoderFactory() {
-  return nullptr;
-}
-
 // Create a new instance of PeerConnectionFactoryInterface.
 //
 // This method relies on the thread it's called on as the "signaling thread"