Add Opus-only audio codec factories

Many WebRTC users need only Opus, and no other audio codecs. This
makes it convenient for them to do the right thing.

To prove that the new factories work, use them in
PeerConnectionEndToEndTest.

Bug: webrtc:11130
Change-Id: I2c2450ba0fb33ef3b50da8f6cd325cad6b1e59a6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/160648
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29921}

api/audio_codecs/builtin_audio_decoder_factory.h

@@ -17,6 +17,10 @@
 namespace webrtc {
 
 // Creates a new factory that can create the built-in types of audio decoders.
+// Note: This will link with all the code implementing those codecs, so if you
+// only need a subset of the codecs, consider using
+// CreateAudioDecoderFactory<...codecs listed here...>() or
+// CreateOpusAudioDecoderFactory() instead.
 rtc::scoped_refptr<AudioDecoderFactory> CreateBuiltinAudioDecoderFactory();
 
 }  // namespace webrtc