Add ToString method for AudioProcessing::Config

Bug: webrtc:10138
Change-Id: Icc0b40596867fd6fb8c1de5ca209dbafb504e50d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/147726
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28745}

modules/audio_processing/include/audio_processing.h

@@ -370,6 +370,8 @@ class AudioProcessing : public rtc::RefCountInterface {
       }
       return *this;
     }
+
+    std::string ToString() const;
   };
 
   // TODO(mgraczyk): Remove once all methods that use ChannelLayout are gone.