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/Classes/VideoToolbox/RTCVideoDecoderH264.mm

@@ -78,7 +78,7 @@ void decompressionOutputCallback(void *decoderRef,
 
 - (instancetype)init {
   if (self = [super init]) {
-#if defined(WEBRTC_IOS)
+#if defined(WEBRTC_IOS) && !defined(RTC_APPRTCMOBILE_BROADCAST_EXTENSION)
     [RTCUIApplicationStatusObserver prepareForUse];
     _error = noErr;
 #endif
@@ -113,7 +113,7 @@ void decompressionOutputCallback(void *decoderRef,
     return WEBRTC_VIDEO_CODEC_ERROR;
   }
 
-#if defined(WEBRTC_IOS)
+#if defined(WEBRTC_IOS) && !defined(RTC_APPRTCMOBILE_BROADCAST_EXTENSION)
   if (![[RTCUIApplicationStatusObserver sharedInstance] isApplicationActive]) {
     // Ignore all decode requests when app isn't active. In this state, the
     // hardware decoder has been invalidated by the OS.