Handle iOS devices with no rear-facing camera

talk/app/webrtc/objc/public/RTCAVFoundationVideoSource.h

 /*
  * libjingle
  * Copyright 2015 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
  *
  *  1. Redistributions of source code must retain the above copyright notice,
  *     this list of conditions and the following disclaimer.
  *  2. Redistributions in binary form must reproduce the above copyright notice,
@@ skipping 23 matching lines @@
 // RTCAVFoundationVideoSource is a video source that uses
 // webrtc::AVFoundationVideoCapturer. We do not currently provide a wrapper for
 // that capturer because cricket::VideoCapturer is not ref counted and we cannot
 // guarantee its lifetime. Instead, we expose its properties through the ref
 // counted video source interface.
 @interface RTCAVFoundationVideoSource : RTCVideoSource
 
 - (instancetype)initWithFactory:(RTCPeerConnectionFactory*)factory
                     constraints:(RTCMediaConstraints*)constraints;
 
+// Returns whether rear-facing camera is available for use.
+@property(nonatomic, readonly) BOOL canUseBackCamera;
 // Switches the camera being used (either front or back).
 @property(nonatomic, assign) BOOL useBackCamera;
 // Returns the active capture session.
 @property(nonatomic, readonly) AVCaptureSession* captureSession;
 
 @end