Add AVFoundation video capture support to Mac objc SDK (based on iOS)

The AppRTCDemo app on Mac OSX does not show or send local video streams,
as ACFoundation capture session is not compiled in or implemented in
the appropriate places.  This is the first part of a two-part patch
that implements local capture on the Mac for AppRTCDemo

P.S. This is my first patch to WebRTC.   I didn't see any relevant tests, but I could write some if you can point me at a location. Also, I don't have access to the automated tests (I don't think)

BUG=webrtc:3417

Review-Url: https://codereview.webrtc.org/2046863004
Cr-Commit-Position: refs/heads/master@{#13080}

webrtc/sdk/objc/Framework/Classes/RTCPeerConnectionFactory.mm

@@ -11,9 +11,7 @@
 #import "RTCPeerConnectionFactory+Private.h"
 
 #import "NSString+StdString.h"
-#if defined(WEBRTC_IOS)
 #import "RTCAVFoundationVideoSource+Private.h"
-#endif
 #import "RTCAudioTrack+Private.h"
 #import "RTCMediaStream+Private.h"
 #import "RTCPeerConnection+Private.h"
@@ -54,12 +52,8 @@
 
 - (RTCAVFoundationVideoSource *)avFoundationVideoSourceWithConstraints:
     (nullable RTCMediaConstraints *)constraints {
-#if defined(WEBRTC_IOS)
   return [[RTCAVFoundationVideoSource alloc] initWithFactory:self
                                                  constraints:constraints];
-#else
-  return nil;
-#endif
 }
 
 - (RTCAudioTrack *)audioTrackWithTrackId:(NSString *)trackId {