Added the API to create an RTCRtpSender to the Objective C wrapper.

Objective C applications can now create new RTCRtpSenders and change their tracks, which gives them more fine grained control than MediaStreams.

BUG=

Review-Url: https://codereview.webrtc.org/1888633002
Cr-Commit-Position: refs/heads/master@{#12570}

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMediaStreamTrack.h

@@ -22,6 +22,9 @@ typedef NS_ENUM(NSInteger, RTCMediaStreamTrackState) {
 
 NS_ASSUME_NONNULL_BEGIN
 
+RTC_EXTERN NSString * const kRTCMediaStreamTrackKindAudio;
+RTC_EXTERN NSString * const kRTCMediaStreamTrackKindVideo;
+
 RTC_EXPORT
 @interface RTCMediaStreamTrack : NSObject
 

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h

@@ -171,6 +171,16 @@ RTC_EXPORT
 
 @end
 
+@interface RTCPeerConnection (Media)
+
+/**
+ * Create an RTCRtpSender with the specified kind and media stream ID.
+ * See RTCMediaStreamTrack.h for available kinds.
+ */
+- (RTCRtpSender *)senderWithKind:(NSString *)kind streamId:(NSString *)streamId;
+
+@end
+
 @interface RTCPeerConnection (DataChannel)
 
 /** Create a new data channel with the given label and configuration. */

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCRtpSender.h

@@ -19,22 +19,20 @@ NS_ASSUME_NONNULL_BEGIN
 RTC_EXPORT
 @protocol RTCRtpSender <NSObject>
 
+/** A unique identifier for this sender. */
+@property(nonatomic, readonly) NSString *senderId;
+
 /** The currently active RTCRtpParameters, as defined in
  *  https://www.w3.org/TR/webrtc/#idl-def-RTCRtpParameters.
  */
-@property(nonatomic, readonly) RTCRtpParameters *parameters;
+@property(nonatomic, copy) RTCRtpParameters *parameters;
 
 /** The RTCMediaStreamTrack associated with the sender.
  *  Note: reading this property returns a new instance of
  *  RTCMediaStreamTrack. Use isEqual: instead of == to compare
  *  RTCMediaStreamTrack instances.
  */
-@property(nonatomic, readonly) RTCMediaStreamTrack *track;
-
-/** Set the new RTCRtpParameters to be used by the sender.
- *  Returns YES if the new parameters were applied, NO otherwise.
- */
-- (BOOL)setParameters:(RTCRtpParameters *)parameters;
+@property(nonatomic, copy, nullable) RTCMediaStreamTrack *track;
 
 @end