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/include/audio_processing.h

@@ -488,6 +488,7 @@ class RTC_EXPORT AudioProcessing : public rtc::RefCountInterface {
   // rate and number of channels) have changed. Passing updated parameters
   // directly to |ProcessStream()| and |ProcessReverseStream()| is permissible.
   // If the parameters are known at init-time though, they may be provided.
+  // TODO(webrtc:5298): Change to return void.
   virtual int Initialize() = 0;
 
   // The int16 interfaces require: