AudioDecoder: Remove the default implementation of SampleRateHz

webrtc/modules/audio_coding/codecs/audio_decoder.h

Index: webrtc/modules/audio_coding/codecs/audio_decoder.h
diff --git a/webrtc/modules/audio_coding/codecs/audio_decoder.h b/webrtc/modules/audio_coding/codecs/audio_decoder.h
index 7b7047f7de6d2eb34ab2835e49089760b9d0b5ef..13581bc247705a740b04e1d9a1cac73e454b3fa3 100644
--- a/webrtc/modules/audio_coding/codecs/audio_decoder.h
+++ b/webrtc/modules/audio_coding/codecs/audio_decoder.h
@@ -95,12 +95,7 @@ class AudioDecoder {
 
   // Returns the actual sample rate of the decoder's output. This value may not
   // change during the lifetime of the decoder.
-  // NOTE: For now, this has a default implementation that returns an unusable
-  // value (-1). That default implementation will go away soon, and at the same
-  // time callers will start relying on the return value, so make sure you
-  // override it with something that returns a correct value!
-  // TODO(kwiberg): Remove the default implementation.
-  virtual int SampleRateHz() const;
+  virtual int SampleRateHz() const = 0;
 
   // The number of channels in the decoder's output. This value may not change
   // during the lifetime of the decoder.