Support building WebRTC without audio and video for IOS

webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCPeerConnectionFactory+Video.mm

+/*
+ *  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.
+ */
+
+#import "RTCPeerConnectionFactory+Private.h"
+
+#import "NSString+StdString.h"
+#import "RTCAVFoundationVideoSource+Private.h"
+#import "RTCAudioSource+Private.h"
+#import "RTCAudioTrack+Private.h"
+#import "RTCMediaConstraints+Private.h"
+#import "RTCMediaStream+Private.h"
+#import "RTCPeerConnection+Private.h"
+#import "RTCVideoSource+Private.h"
+#import "RTCVideoTrack+Private.h"
+#import "WebRTC/RTCLogging.h"
+
+#include "Video/objcvideotracksource.h"
+#include "VideoToolbox/videocodecfactory.h"
+#include "webrtc/api/videosourceproxy.h"
+
+@implementation RTCPeerConnectionFactory (Video)

[tkchin_webrtc, 2017/06/20 21:51:24]

ditto: why separate file?

+
+- (RTCAVFoundationVideoSource *)avFoundationVideoSourceWithConstraints:
+        (nullable RTCMediaConstraints *)constraints {
+#ifdef HAVE_RTC_VIDEO
+  return [[RTCAVFoundationVideoSource alloc] initWithFactory:self constraints:constraints];
+#else
+  return nil;
+#endif
+}
+
+- (RTCVideoSource *)videoSource {
+#ifdef HAVE_RTC_VIDEO
+  rtc::scoped_refptr<webrtc::ObjcVideoTrackSource> objcVideoTrackSource(
+      new rtc::RefCountedObject<webrtc::ObjcVideoTrackSource>());
+  return [[RTCVideoSource alloc]
+      initWithNativeVideoSource:webrtc::VideoTrackSourceProxy::Create(_signalingThread.get(),
+                                                                      _workerThread.get(),
+                                                                      objcVideoTrackSource)];
+#else
+  return nil;
+#endif
+}
+
+- (RTCVideoTrack *)videoTrackWithSource:(RTCVideoSource *)source trackId:(NSString *)trackId {
+#ifdef HAVE_RTC_VIDEO
+  return [[RTCVideoTrack alloc] initWithFactory:self source:source trackId:trackId];

[tkchin_webrtc, 2017/06/20 21:51:24]

for the generic case, why do we need to special case it? 
Since we have a thin wrapper, it should just return whatever the C++ impl
returns

+#else
+  return nil;
+#endif
+}
+
+@end