Improved thread checking scheme for iOS.

TBR=zeke

Bug: b/63071036
Change-Id: Iaa6325a8d360f121f82683115c73cc136e174ba6
Reviewed-on: https://chromium-review.googlesource.com/552539
Reviewed-by: Henrik Andreasson <henrika@webrtc.org>
Commit-Queue: Henrik Andreasson <henrika@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18810}

webrtc/modules/audio_device/ios/audio_device_ios.mm

@@ -928,6 +928,10 @@ void AudioDeviceIOS::ShutdownPlayOrRecord() {
   // Close and delete the voice-processing I/O unit.
   audio_unit_.reset();
 
+  // Detach thread checker for the AURemoteIO::IOThread to ensure that the
+  // next session uses a fresh thread id.
+  io_thread_checker_.DetachFromThread();
+
   // Remove audio session notification observers.
   RTCAudioSession* session = [RTCAudioSession sharedInstance];
   [session removeDelegate:audio_session_observer_];