iOS: add type to peer connection local streams

BUG=

Review-Url: https://codereview.webrtc.org/2249173002
Cr-Commit-Position: refs/heads/master@{#13825}
diff --git a/AUTHORS b/AUTHORS
index e9e9d66..6d1ee96 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -21,6 +21,7 @@
 Manish Jethani <manish.jethani@gmail.com>
 Martin Storsjo <martin@martin.st>
 Matthias Liebig <matthias.gcode@gmail.com>
+Maxim Potapov <vopatop.skam@gmail.com>
 Pali Rohar
 Paul Kapustin <pkapustin@gmail.com>
 Rafael Lopez Diez <rafalopezdiez@gmail.com>
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm b/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm
index 428eb24..0af400b 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm
@@ -204,7 +204,7 @@
 
 
 @implementation RTCPeerConnection {
-  NSMutableArray *_localStreams;
+  NSMutableArray<RTCMediaStream *> *_localStreams;
   std::unique_ptr<webrtc::PeerConnectionDelegateAdapter> _observer;
   rtc::scoped_refptr<webrtc::PeerConnectionInterface> _peerConnection;
   BOOL _hasStartedRtcEventLog;
@@ -241,7 +241,7 @@
   return self;
 }
 
-- (NSArray *)localStreams {
+- (NSArray<RTCMediaStream *> *)localStreams {
   return [_localStreams copy];
 }
 
diff --git a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h
index 87d643d..3ea97c3 100644
--- a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h
+++ b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h
@@ -115,7 +115,7 @@
  *  streams being added or removed.
  */
 @property(nonatomic, weak, nullable) id<RTCPeerConnectionDelegate> delegate;
-@property(nonatomic, readonly) NSArray *localStreams;
+@property(nonatomic, readonly) NSArray<RTCMediaStream *> *localStreams;
 @property(nonatomic, readonly, nullable)
     RTCSessionDescription *localDescription;
 @property(nonatomic, readonly, nullable)