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/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];
 }