Add GetLargestGuaranteedMessagePayload to QuicSession/Connection/Generator/Creator to expose the largest message payload that's guaranteed to fit in any packet containing application data.

gfe-relnote: n/a (Only used in Quartc)
PiperOrigin-RevId: 242135946
Change-Id: I52cc763af88d22fc240210e00cf4d4062d0b9c59
diff --git a/quic/quartc/quartc_session.h b/quic/quartc/quartc_session.h
index 35df8b6..29e6306 100644
--- a/quic/quartc/quartc_session.h
+++ b/quic/quartc/quartc_session.h
@@ -51,8 +51,8 @@
   bool SendOrQueueMessage(std::string message);
 
   // Returns largest message payload acceptable in SendQuartcMessage.
-  QuicPacketLength GetLargestMessagePayload() const {
-    return connection()->GetLargestMessagePayload();
+  QuicPacketLength GetCurrentLargestMessagePayload() const {
+    return connection()->GetCurrentLargestMessagePayload();
   }
 
   // Return true if transport support message frame.