AcmReceiver: Ask NetEq to delete all decoders at once instead of one by one

It requires a new NetEq method, but it can no longer fail. And we no
longer need to use AcmReceiver::decoders_, which we're trying to
eliminate.

(This is a re-land of https://codereview.webrtc.org/2342313002.)

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/2348233002
Cr-Commit-Position: refs/heads/master@{#14304}

webrtc/modules/audio_coding/neteq/decoder_database.h

@@ -136,6 +136,9 @@ class DecoderDatabase {
   // Returns kDecoderNotFound or kOK depending on the outcome of the operation.
   virtual int Remove(uint8_t rtp_payload_type);
 
+  // Remove all entries.
+  virtual void RemoveAll();
+
   // Returns a pointer to the DecoderInfo struct for |rtp_payload_type|. If
   // no decoder is registered with that |rtp_payload_type|, NULL is returned.
   virtual const DecoderInfo* GetDecoderInfo(uint8_t rtp_payload_type) const;