Handle iOS devices with no rear-facing camera

talk/app/webrtc/objc/avfoundationvideocapturer.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 38 matching lines @@
     return false;
   }
   bool GetPreferredFourccs(std::vector<uint32_t>* fourccs) override {
     fourccs->push_back(cricket::FOURCC_NV12);
     return true;
   }
 
   // Returns the active capture session.
   AVCaptureSession* GetCaptureSession();
 
+  // Returns whether the rear-facing camera can be used.
+  // e.g. It can't be used because it doesn't exist.
+  bool CanUseBackCamera() const;
+
   // Switches the camera being used (either front or back).
   void SetUseBackCamera(bool useBackCamera);
   bool GetUseBackCamera() const;
 
   // Converts the sample buffer into a cricket::CapturedFrame and signals the
   // frame for capture.
   void CaptureSampleBuffer(CMSampleBufferRef sampleBuffer);
 
  private:
   // Used to signal frame capture on the thread that capturer was started on.
   void SignalFrameCapturedOnStartThread(const cricket::CapturedFrame* frame);
 
   RTCAVFoundationVideoCapturerInternal* _capturer;
   rtc::Thread* _startThread;  // Set in Start(), unset in Stop().
 };  // AVFoundationVideoCapturer
 
 }  // namespace webrtc
 
 #endif  // TALK_APP_WEBRTC_OBJC_AVFOUNDATION_CAPTURER_H_