Avoid race when stopping audio unit on iOS

BUG=webrtc:5993
R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/2079383002 .

Cr-Commit-Position: refs/heads/master@{#13234}

webrtc/modules/audio_device/ios/audio_device_ios.mm

diff --git a/webrtc/modules/audio_device/ios/audio_device_ios.mm b/webrtc/modules/audio_device/ios/audio_device_ios.mm
index 4c1b41d..0a8da0f 100644
--- a/webrtc/modules/audio_device/ios/audio_device_ios.mm
+++ b/webrtc/modules/audio_device/ios/audio_device_ios.mm
@@ -829,6 +829,9 @@
 void AudioDeviceIOS::ShutdownPlayOrRecord() {
   LOGI() << "ShutdownPlayOrRecord";
 
+  // Stop the audio unit to prevent any additional audio callbacks.
+  audio_unit_->Stop();
+
   // Close and delete the voice-processing I/O unit.
   audio_unit_.reset();
 

webrtc/modules/audio_device/ios/voice_processing_audio_unit.mm

diff --git a/webrtc/modules/audio_device/ios/voice_processing_audio_unit.mm b/webrtc/modules/audio_device/ios/voice_processing_audio_unit.mm
index a9d599a..d8805a1 100644
--- a/webrtc/modules/audio_device/ios/voice_processing_audio_unit.mm
+++ b/webrtc/modules/audio_device/ios/voice_processing_audio_unit.mm
@@ -380,6 +380,7 @@
         break;
     }
 
+    RTCLog(@"Disposing audio unit.");
     OSStatus result = AudioComponentInstanceDispose(vpio_unit_);
     if (result != noErr) {
       RTCLogError(@"AudioComponentInstanceDispose failed. Error=%ld.",