NetEq: Ask AudioDecoder for sample rate instead of passing it as an argument

BUG=webrtc:5801
NOTRY=true

Review-Url: https://codereview.webrtc.org/2027993002
Cr-Commit-Position: refs/heads/master@{#13162}
diff --git a/webrtc/modules/audio_coding/acm2/acm_receiver.cc b/webrtc/modules/audio_coding/acm2/acm_receiver.cc
index c15239f..bad484c 100644
--- a/webrtc/modules/audio_coding/acm2/acm_receiver.cc
+++ b/webrtc/modules/audio_coding/acm2/acm_receiver.cc
@@ -242,7 +242,7 @@
     ret_val = neteq_->RegisterPayloadType(neteq_decoder, name, payload_type);
   } else {
     ret_val = neteq_->RegisterExternalDecoder(
-        audio_decoder, neteq_decoder, name, payload_type, sample_rate_hz);
+        audio_decoder, neteq_decoder, name, payload_type);
   }
   if (ret_val != NetEq::kOK) {
     LOG(LERROR) << "AcmReceiver::AddCodec " << acm_codec_id