Make PeerConnection::session_id_ const and readable from any thread.

Going forward, we'll need to read this value from other threads than
signaling, so I've moved the initialization into the constructor.

Bug: none
Change-Id: I56b00d38c86788cbab9a2055719074ea48f4750f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/213185
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33613}
diff --git a/pc/peer_connection.h b/pc/peer_connection.h
index 58424db..aa46feb 100644
--- a/pc/peer_connection.h
+++ b/pc/peer_connection.h
@@ -271,7 +271,6 @@
   rtc::Thread* worker_thread() const final { return context_->worker_thread(); }
 
   std::string session_id() const override {
-    RTC_DCHECK_RUN_ON(signaling_thread());
     return session_id_;
   }
 
@@ -670,7 +669,7 @@
   rtc::scoped_refptr<RTCStatsCollector> stats_collector_
       RTC_GUARDED_BY(signaling_thread());
 
-  std::string session_id_ RTC_GUARDED_BY(signaling_thread());
+  const std::string session_id_;
 
   std::unique_ptr<JsepTransportController>
       transport_controller_;  // TODO(bugs.webrtc.org/9987): Accessed on both