Support delayed AudioUnit initialization.

webrtc/modules/audio_device/ios/objc/RTCAudioSessionDelegateAdapter.mm

 /*
  *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 58 matching lines @@
 
 - (void)audioSessionMediaServicesWereReset:(RTCAudioSession *)session {
 }
 
 - (void)audioSessionShouldConfigure:(RTCAudioSession *)session {
 }
 
 - (void)audioSessionShouldUnconfigure:(RTCAudioSession *)session {
 }
 
+- (void)audioSessionDidConfigure:(RTCAudioSession *)session {
+  _observer->OnConfiguredForWebRTC();
+}
+
 @end