audio_processing: Create now returns a pointer to the object

Affects
* NS
* AGC
* AEC

BUG=441
TESTED=locally on Linux and trybots
R=kwiberg@webrtc.org

Review URL: https://codereview.webrtc.org/1175903002.

Cr-Commit-Position: refs/heads/master@{#9411}

webrtc/modules/audio_processing/gain_control_impl.cc

@@ -301,14 +301,7 @@ int GainControlImpl::Initialize() {
 }
 
 void* GainControlImpl::CreateHandle() const {
-  Handle* handle = NULL;
-  if (WebRtcAgc_Create(&handle) != apm_->kNoError) {
-    handle = NULL;
-  } else {
-    assert(handle != NULL);
-  }
-
-  return handle;
+  return WebRtcAgc_Create();
 }
 
 void GainControlImpl::DestroyHandle(void* handle) const {