Address tkchin's comments on RTCCameraVideoCapturer.

webrtc/sdk/objc/Framework/Classes/RTCCameraVideoCapturer.m

 /*
  *  Copyright 2017 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 343 matching lines @@
     kCVPixelBufferPixelFormatTypeKey : @(kCVPixelFormatType_420YpCbCr8BiPlanarFullRange)
   };
   videoDataOutput.alwaysDiscardsLateVideoFrames = NO;
   [videoDataOutput setSampleBufferDelegate:self queue:self.frameQueue];
   _videoDataOutput = videoDataOutput;
 }
 
 #pragma mark - Private, called inside capture queue
 
 - (void)updateDeviceCaptureFormat:(AVCaptureDeviceFormat *)format fps:(int)fps {
+  NSAssert([RTCDispatcher isOnQueueForType:RTCDispatcherTypeCaptureSession],
+           @"updateDeviceCaptureFormat must be called on the capture queue.");
   @try {
     _currentDevice.activeFormat = format;
     _currentDevice.activeVideoMinFrameDuration = CMTimeMake(1, fps);
   } @catch (NSException *exception) {
     RTCLogError(@"Failed to set active format!\n User info:%@", exception.userInfo);
     return;
   }
 }
 
 - (void)reconfigureCaptureSessionInput {
+  NSAssert([RTCDispatcher isOnQueueForType:RTCDispatcherTypeCaptureSession],
+           @"reconfigureCaptureSessionInput must be called on the capture queue.");
   NSError *error = nil;
   AVCaptureDeviceInput *input =
       [AVCaptureDeviceInput deviceInputWithDevice:_currentDevice error:&error];
   if (!input) {
     RTCLogError(@"Failed to create front camera input: %@", error.localizedDescription);
     return;
   }
   for (AVCaptureDeviceInput *oldInput in [_captureSession.inputs copy]) {
     [_captureSession removeInput:oldInput];
   }
   if ([_captureSession canAddInput:input]) {
     [_captureSession addInput:input];
   } else {
     RTCLogError(@"Cannot add camera as an input to the session.");
     return;
   }
 }
 
 - (void)updateOrientation {
+  NSAssert([RTCDispatcher isOnQueueForType:RTCDispatcherTypeCaptureSession],
+           @"updateOrientation must be called on the capture queue.");
 #if TARGET_OS_IPHONE
   BOOL usingFrontCamera = _currentDevice.position == AVCaptureDevicePositionFront;
   switch ([UIDevice currentDevice].orientation) {
     case UIDeviceOrientationPortrait:
       _rotation = RTCVideoRotation_90;
       break;
     case UIDeviceOrientationPortraitUpsideDown:
       _rotation = RTCVideoRotation_270;
       break;
     case UIDeviceOrientationLandscapeLeft:
       _rotation = usingFrontCamera ? RTCVideoRotation_180 : RTCVideoRotation_0;
       break;
     case UIDeviceOrientationLandscapeRight:
       _rotation = usingFrontCamera ? RTCVideoRotation_0 : RTCVideoRotation_180;
       break;
     case UIDeviceOrientationFaceUp:
     case UIDeviceOrientationFaceDown:
     case UIDeviceOrientationUnknown:
       // Ignore.
       break;
   }
 #endif
 }
 
 @end