Adds audio session status to logs for each valid audio route change on iOS

BUG=b/30944297
NOTRY=TRUE

Review-Url: https://codereview.webrtc.org/2355393005
Cr-Commit-Position: refs/heads/master@{#14355}

webrtc/modules/audio_device/ios/audio_device_ios.mm

@@ -491,6 +491,7 @@ void AudioDeviceIOS::HandleValidRouteChange() {
   RTC_DCHECK(thread_checker_.CalledOnValidThread());
 
   RTCAudioSession* session = [RTCAudioSession sharedInstance];
+  RTCLog(@"%@", session);
   HandleSampleRateChange(session.sampleRate);
 }