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}
diff --git a/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.h b/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.h
index a2064df..0c03295 100644
--- a/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.h
+++ b/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.h
@@ -12,6 +12,7 @@
 
 #import <Foundation/Foundation.h>
 
+NS_EXTENSION_UNAVAILABLE_IOS("Application status not available in app extensions.")
 @interface RTCUIApplicationStatusObserver : NSObject
 
 + (instancetype)sharedInstance;