Stop using "delete connection()" in QuicSession classes and use DeleteConnection() instead, which prevents double deletion and sets connection_ to nullptr.

gfe-relnote: n/a (Set deleted pointer to nullptr)
PiperOrigin-RevId: 284764494
Change-Id: I6d7f36768e14f06b9cd8267ac74d22d189463579
diff --git a/quic/core/quic_session.h b/quic/core/quic_session.h
index c6861f9..eb9cdfe 100644
--- a/quic/core/quic_session.h
+++ b/quic/core/quic_session.h
@@ -626,6 +626,10 @@
   // indicated by |unidirectional|.
   QuicStreamId GetLargestPeerCreatedStreamId(bool unidirectional) const;
 
+  // Deletes the connection and sets it to nullptr, so calling it mulitiple
+  // times is safe.
+  void DeleteConnection();
+
  private:
   friend class test::QuicSessionPeer;