NetEq: Drop unnecessary dependency on the audio decoder implementations

BUG=webrtc:8396

Change-Id: I7524dae93b43b656a13fdd535e48373bc29b405e
Reviewed-on: https://webrtc-review.googlesource.com/10804
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20310}

modules/audio_coding/neteq/decoder_database.h

@@ -19,7 +19,7 @@
 #include "api/audio_codecs/audio_format.h"
 #include "common_types.h"  // NOLINT(build/include)  // NULL
 #include "modules/audio_coding/codecs/cng/webrtc_cng.h"
-#include "modules/audio_coding/neteq/audio_decoder_impl.h"
+#include "modules/audio_coding/neteq/neteq_decoder_enum.h"
 #include "modules/audio_coding/neteq/packet.h"
 #include "rtc_base/constructormagic.h"
 #include "rtc_base/scoped_ref_ptr.h"
@@ -55,6 +55,10 @@ class DecoderDatabase {
     DecoderInfo(DecoderInfo&&);
     ~DecoderInfo();
 
+    // Was this info object created with a specification that allows us to
+    // actually produce a decoder?
+    bool CanGetDecoder() const;
+
     // Get the AudioDecoder object, creating it first if necessary.
     AudioDecoder* GetDecoder() const;