Make the old GetStats interface on AudioProcessorInterface impure.

This is the first step towards removing this function from the interface. Once all implementers of the interface remove their implementations this can be removed.

Bug: webrtc:8572
Change-Id: Ia8f7f1b6949a482787df67b193d4cf999142e06e
Reviewed-on: https://webrtc-review.googlesource.com/27620
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21271}

api/mediastreaminterface.cc

@@ -9,12 +9,21 @@
  */
 
 #include "api/mediastreaminterface.h"
+#include "rtc_base/checks.h"
+#include "rtc_base/logging.h"
 
 namespace webrtc {
 
 const char MediaStreamTrackInterface::kVideoKind[] = "video";
 const char MediaStreamTrackInterface::kAudioKind[] = "audio";
 
+void AudioProcessorInterface::GetStats(AudioProcessorStats* /*stats*/) {
+  RTC_NOTREACHED() << "Old-style GetStats() is called but it has no "
+                   << "implementation.";
+  RTC_LOG(LS_ERROR) << "Old-style GetStats() is called but it has no "
+                    << "implementation.";
+}
+
 // TODO(ivoc): Remove this when the function becomes pure virtual.
 AudioProcessorInterface::AudioProcessorStatistics
 AudioProcessorInterface::GetStats(bool /*has_remote_tracks*/) {

api/mediastreaminterface.h

@@ -258,7 +258,7 @@ class AudioProcessorInterface : public rtc::RefCountInterface {
   };
 
   // Get audio processor statistics.
-  virtual void GetStats(AudioProcessorStats* stats) = 0;
+  virtual void GetStats(AudioProcessorStats* stats);
 
   // Get audio processor statistics. The |has_remote_tracks| argument should be
   // set if there are active remote tracks (this would usually be true during