Make AudioProcessing::GetConfig() pure virtual

BUG=chromium:708475
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_rel_ng

Review-Url: https://codereview.webrtc.org/2796223004
Cr-Commit-Position: refs/heads/master@{#17581}
diff --git a/webrtc/modules/audio_processing/include/audio_processing.h b/webrtc/modules/audio_processing/include/audio_processing.h
index 99e4b69..cc6772b 100644
--- a/webrtc/modules/audio_processing/include/audio_processing.h
+++ b/webrtc/modules/audio_processing/include/audio_processing.h
@@ -560,9 +560,7 @@
   virtual VoiceDetection* voice_detection() const = 0;
 
   // Returns the last applied configuration.
-  // TODO(henrik.lundin) Make this method pure virtual when downstream
-  // dependencies have been fixed.
-  virtual AudioProcessing::Config GetConfig() const;
+  virtual AudioProcessing::Config GetConfig() const = 0;
 
   enum Error {
     // Fatal errors.