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/webrtc.gni b/webrtc.gni
index 123e2c2..a71d8e7 100644
--- a/webrtc.gni
+++ b/webrtc.gni
@@ -151,6 +151,12 @@
 
   # Disable this to build without support for built-in software codecs.
   rtc_use_builtin_sw_codecs = true
+
+  if (is_ios) {
+    # Build broadcast extension in AppRTCMobile for iOS. This results in the
+    # binary only running on iOS 11+, which is why it is disabled by default.
+    rtc_apprtcmobile_broadcast_extension = false
+  }
 }
 
 if (!build_with_mozilla) {