Add flag to QuicSession indicating that it has been configured

This CL adds a flag to QuicSession that indicates when the QuicSession has been
configured. This needed for follow-on work where IETF QUIC is limited as to the
frames that can be sent _until_ the transport config has been performed.

QuicSession test has been modified to A) artificially perform a configuration
and B) check at test termination that it has been configured.

gfe-relnote: N/A not significant. Other changes all to tests.
PiperOrigin-RevId: 264362154
Change-Id: I1467cd4ed4531c01950d3fb938637e8633de815e
diff --git a/quic/core/quic_session.h b/quic/core/quic_session.h
index 6ab56f9..8216bbd 100644
--- a/quic/core/quic_session.h
+++ b/quic/core/quic_session.h
@@ -431,6 +431,8 @@
     return use_http2_priority_write_scheduler_;
   }
 
+  bool is_configured() const { return is_configured_; }
+
  protected:
   using StreamMap = QuicSmallMap<QuicStreamId, std::unique_ptr<QuicStream>, 10>;
 
@@ -724,6 +726,10 @@
   // If true, write_blocked_streams_ uses HTTP2 (tree-style) priority write
   // scheduler.
   bool use_http2_priority_write_scheduler_;
+
+  // Initialized to false. Set to true when the session has been properly
+  // configured and is ready for general operation.
+  bool is_configured_;
 };
 
 }  // namespace quic