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_test.cc b/quic/quartc/quartc_session_test.cc
index fc80c8b..31abd57 100644
--- a/quic/quartc/quartc_session_test.cc
+++ b/quic/quartc/quartc_session_test.cc
@@ -230,12 +230,12 @@
 
     // Send message of maximum allowed length.
     std::string message_max_long =
-        std::string(server_peer_->GetLargestMessagePayload(), 'A');
+        std::string(server_peer_->GetCurrentLargestMessagePayload(), 'A');
     ASSERT_TRUE(server_peer_->SendOrQueueMessage(message_max_long));
 
     // Send long message which should fail.
     std::string message_too_long =
-        std::string(server_peer_->GetLargestMessagePayload() + 1, 'B');
+        std::string(server_peer_->GetCurrentLargestMessagePayload() + 1, 'B');
     ASSERT_FALSE(server_peer_->SendOrQueueMessage(message_too_long));
 
     // Wait for peer 2 to receive message.