Refactor some ObjC API init methods.

webrtc/api/objc/RTCMediaStream+Private.h

Index: webrtc/api/objc/RTCMediaStream+Private.h
diff --git a/webrtc/api/objc/RTCMediaStream+Private.h b/webrtc/api/objc/RTCMediaStream+Private.h
index 4c832881909c87e9fae0d739b04e767ffee56a45..b03b091c11f3e7952320d1bbac1747a2d4a11e33 100644
--- a/webrtc/api/objc/RTCMediaStream+Private.h
+++ b/webrtc/api/objc/RTCMediaStream+Private.h
@@ -23,6 +23,10 @@ NS_ASSUME_NONNULL_BEGIN
 @property(nonatomic, readonly)
     rtc::scoped_refptr<webrtc::MediaStreamInterface> nativeMediaStream;
 
+/** Initialize an RTCMediaStream with an id. */
+- (instancetype)initWithFactory:(RTCPeerConnectionFactory *)factory
+                       streamId:(NSString *)streamId;
+
 /** Initialize an RTCMediaStream from a native MediaStreamInterface. */
 - (instancetype)initWithNativeMediaStream:
     (rtc::scoped_refptr<webrtc::MediaStreamInterface>)nativeMediaStream;