RtpReceiverInterface::stream_ids() added.

This is the first step to removing streams from third_party/webrtc.
RtpReceiverInterface::streams() will have to be removed separately.
See https://crbug.com/webrtc/9480 for more information.

Bug: webrtc:9480
Change-Id: I6f9e6ddcda5e2245cc601d2cc6205b7b363f73ef
Reviewed-on: https://webrtc-review.googlesource.com/86840
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23929}
diff --git a/api/rtpreceiverinterface.h b/api/rtpreceiverinterface.h
index 1801dc1..b2499b4 100644
--- a/api/rtpreceiverinterface.h
+++ b/api/rtpreceiverinterface.h
@@ -93,8 +93,12 @@
   virtual rtc::scoped_refptr<MediaStreamTrackInterface> track() const = 0;
   // The list of streams that |track| is associated with. This is the same as
   // the [[AssociatedRemoteMediaStreams]] internal slot in the spec.
-  // https://w3c.github.io/webrtc-pc/#dfn-x%5B%5Bassociatedremotemediastreams%5D%5D
+  // https://w3c.github.io/webrtc-pc/#dfn-associatedremotemediastreams
   // TODO(hbos): Make pure virtual as soon as Chromium's mock implements this.
+  // TODO(https://crbug.com/webrtc/9480): Remove streams() in favor of
+  // stream_ids() as soon as downstream projects are no longer dependent on
+  // stream objects.
+  virtual std::vector<std::string> stream_ids() const;
   virtual std::vector<rtc::scoped_refptr<MediaStreamInterface>> streams() const;
 
   // Audio or video receiver?