Handle iOS devices with no rear-facing camera

talk/app/webrtc/objc/RTCAVFoundationVideoSource.mm

 /*
  * 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 28 matching lines @@
 - (instancetype)initWithFactory:(RTCPeerConnectionFactory*)factory
                     constraints:(RTCMediaConstraints*)constraints {
   NSParameterAssert(factory);
   std::unique_ptr<webrtc::AVFoundationVideoCapturer> capturer;
   capturer.reset(new webrtc::AVFoundationVideoCapturer());
   rtc::scoped_refptr<webrtc::VideoTrackSourceInterface> source =
       factory.nativeFactory->CreateVideoSource(capturer.release(), constraints.constraints);
   return [super initWithMediaSource:source];
 }
 
+- (BOOL)canUseBackCamera {
+  return self.capturer->CanUseBackCamera();
+}
+
 - (BOOL)useBackCamera {
   return self.capturer->GetUseBackCamera();
 }
 
 - (void)setUseBackCamera:(BOOL)useBackCamera {
   self.capturer->SetUseBackCamera(useBackCamera);
 }
 
 - (AVCaptureSession*)captureSession {
   return self.capturer->GetCaptureSession();
 }
 
 - (webrtc::AVFoundationVideoCapturer*)capturer {
   cricket::VideoCapturer* capturer = self.videoSource->GetVideoCapturer();
   // This should be safe because no one should have changed the underlying video
   // source.
   webrtc::AVFoundationVideoCapturer* foundationCapturer =
       static_cast<webrtc::AVFoundationVideoCapturer*>(capturer);
   return foundationCapturer;
 }
 
 @end