Avoid race when stopping audio unit on iOS

webrtc/modules/audio_device/ios/voice_processing_audio_unit.mm

 /*
  *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 362 matching lines @@
         FALLTHROUGH();
       case kInitialized:
         Uninitialize();
         break;
       case kUninitialized:
         FALLTHROUGH();
       case kInitRequired:
         break;
     }
 
+    RTCLog(@"Disposing audio unit.");
     OSStatus result = AudioComponentInstanceDispose(vpio_unit_);
     if (result != noErr) {
       RTCLogError(@"AudioComponentInstanceDispose failed. Error=%ld.",
                   (long)result);
     }
     vpio_unit_ = nullptr;
   }
 }
 
 }  // namespace webrtc