Cleanup of the APM creation

This CL removes the possibility that APM cannot be created, i.e., that
the create method can return nullptr. That was already the case
implicitly but this CL makes that behavior explicit.


Bug: webrtc:5298
Change-Id: I2706ea538c9d1b4bcd65faecab637640a209a4dc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/183101
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32029}

modules/audio_processing/audio_processing_impl.h

@@ -140,7 +140,7 @@ class AudioProcessingImpl : public AudioProcessing {
 
  protected:
   // Overridden in a mock.
-  virtual int InitializeLocked()
+  virtual void InitializeLocked()
       RTC_EXCLUSIVE_LOCKS_REQUIRED(mutex_render_, mutex_capture_);
 
  private: