Handle iOS devices with no rear-facing camera

webrtc/api/objc/RTCAVFoundationVideoSource.h

 /*
  *  Copyright 2015 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 10 matching lines @@
  * 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:(nullable 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
 
 
 NS_ASSUME_NONNULL_END