Add an example app for iOS native API.

Demonstrates how to use the iOS native API to wrap components into
C++ classes.

This CL also introduces a native API wrapper for the capturer.

The C++ code is forked from the corresponding CL for Android at
https://webrtc-review.googlesource.com/c/src/+/60540

Bug: webrtc:8832
Change-Id: I12d9f30e701c0222628e329218f6d5bfca26e6e0
Reviewed-on: https://webrtc-review.googlesource.com/61422
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22484}

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

@@ -46,6 +46,10 @@ const int64_t kNanosecondsPerSecond = 1000000000;
 @synthesize frameQueue = _frameQueue;
 @synthesize captureSession = _captureSession;
 
+- (instancetype)init {
+  return [self initWithDelegate:nil captureSession:[[AVCaptureSession alloc] init]];
+}
+
 - (instancetype)initWithDelegate:(__weak id<RTCVideoCapturerDelegate>)delegate {
   return [self initWithDelegate:delegate captureSession:[[AVCaptureSession alloc] init]];
 }

sdk/objc/Framework/Classes/PeerConnection/RTCVideoCapturer.m

@@ -15,7 +15,6 @@
 @synthesize delegate = _delegate;
 
 - (instancetype)initWithDelegate:(id<RTCVideoCapturerDelegate>)delegate {
-  NSAssert(delegate != nil, @"delegate cannot be nil");
   if (self = [super init]) {
     _delegate = delegate;
   }