[Unified Plan] Implement FiredDirection for RtpTransceiver

Bug: webrtc:9236
Change-Id: Ib5a8215f3762f35b68d2a285c7d676f93f1212c5
Reviewed-on: https://webrtc-review.googlesource.com/88921
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24010}

api/rtptransceiverinterface.h

@@ -107,6 +107,15 @@ class RtpTransceiverInterface : public rtc::RefCountInterface {
   // https://w3c.github.io/webrtc-pc/#dom-rtcrtptransceiver-currentdirection
   virtual absl::optional<RtpTransceiverDirection> current_direction() const = 0;
 
+  // An internal slot designating for which direction the relevant
+  // PeerConnection events have been fired. This is to ensure that events like
+  // OnAddTrack only get fired once even if the same session description is
+  // applied again.
+  // Exposed in the public interface for use by Chromium.
+  virtual absl::optional<RtpTransceiverDirection> fired_direction() const {
+    return absl::nullopt;
+  }
+
   // The Stop method irreversibly stops the RtpTransceiver. The sender of this
   // transceiver will no longer send, the receiver will no longer receive.
   // https://w3c.github.io/webrtc-pc/#dom-rtcrtptransceiver-stop