Broadcast extension for AppRTCMobile on iOS

This provides an environment for testing out using WebRTC from an iOS
extension. It implements a ReplayKit broadcast extension for live
streaming games and screensharing.

The extension is only supported on iOS 11+ and is guarded by a build
flag.

Bug: webrtc:9335
Change-Id: Id218d6c73ef7599f5953c5a1e0e62e5d0dc4f10b
Reviewed-on: https://webrtc-review.googlesource.com/80000
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23504}

sdk/objc/Framework/Headers/WebRTC/RTCEAGLVideoView.h

@@ -28,6 +28,7 @@ RTC_EXPORT
  * bounds using OpenGLES 2.0 or OpenGLES 3.0.
  */
 RTC_EXPORT
+NS_EXTENSION_UNAVAILABLE_IOS("Rendering not available in app extensions.")
 @interface RTCEAGLVideoView : UIView <RTCVideoRenderer>
 
 @property(nonatomic, weak) id<RTCVideoViewDelegate> delegate;