Add new view that renders local video using AVCaptureLayerPreview.

talk/app/webrtc/objc/avfoundationvideocapturer.mm

Index: talk/app/webrtc/objc/avfoundationvideocapturer.mm
diff --git a/talk/app/webrtc/objc/avfoundationvideocapturer.mm b/talk/app/webrtc/objc/avfoundationvideocapturer.mm
index e1b0f88fb6ee590b405af970fefbef4db921bd11..34c05dececdbb808c5cc815724235acaaa065708 100644
--- a/talk/app/webrtc/objc/avfoundationvideocapturer.mm
+++ b/talk/app/webrtc/objc/avfoundationvideocapturer.mm
@@ -54,6 +54,7 @@ static dispatch_queue_t kBackgroundQueue = nil;
     <AVCaptureVideoDataOutputSampleBufferDelegate>
 
 @property(nonatomic, readonly) AVCaptureSession* captureSession;
+@property(nonatomic, readonly) dispatch_queue_t captureSessionQueue;
 @property(nonatomic, readonly) BOOL isRunning;
 @property(nonatomic, assign) BOOL useBackCamera;  // Defaults to NO.
 
@@ -117,6 +118,10 @@ static dispatch_queue_t kBackgroundQueue = nil;
   _capturer = nullptr;
 }
 
+- (dispatch_queue_t)captureSessionQueue {
+  return kBackgroundQueue;
+}
+
 - (void)setUseBackCamera:(BOOL)useBackCamera {
   if (_useBackCamera == useBackCamera) {
     return;
@@ -371,6 +376,10 @@ bool AVFoundationVideoCapturer::GetUseBackCamera() const {
   return _capturer.useBackCamera;
 }
 
+dispatch_queue_t AVFoundationVideoCapturer::GetCaptureSessionQueue() {
+  return _capturer.captureSessionQueue;
+}
+
 void AVFoundationVideoCapturer::CaptureSampleBuffer(
     CMSampleBufferRef sampleBuffer) {
   if (CMSampleBufferGetNumSamples(sampleBuffer) != 1 ||