Changes name of RtpTransceiverInit's stream_labels to stream_ids.

The naming convention according to the spec is stream id, not stream
labels.Changing things now to be spec compliant, before it is widely
used. This also includes changes to objective C wrapper code to be in
sync with the change.

Bug: webrtc:7932
Change-Id: I5705e6d8a647aaeed860316466a7320132f24b00
Reviewed-on: https://webrtc-review.googlesource.com/59301
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Seth Hampson <shampson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22316}

sdk/objc/Framework/Classes/PeerConnection/RTCMediaStream.mm

@@ -43,7 +43,7 @@
 }
 
 - (NSString *)streamId {
-  return [NSString stringForStdString:_nativeMediaStream->label()];
+  return [NSString stringForStdString:_nativeMediaStream->id()];
 }
 
 - (void)addAudioTrack:(RTCAudioTrack *)audioTrack {