Add some QUIC DCHECKs

These assumptions were found when trying to debug crbug/1117201

Debug-only change

PiperOrigin-RevId: 328638627
Change-Id: Ic563044d9e5d7be8965a62b126215c066d0b400f
diff --git a/quic/core/quic_packet_creator.h b/quic/core/quic_packet_creator.h
index 2c1ff57..5edcff3 100644
--- a/quic/core/quic_packet_creator.h
+++ b/quic/core/quic_packet_creator.h
@@ -282,9 +282,7 @@
   void SetClientConnectionId(QuicConnectionId client_connection_id);
 
   // Sets the encryption level that will be applied to new packets.
-  void set_encryption_level(EncryptionLevel level) {
-    packet_.encryption_level = level;
-  }
+  void set_encryption_level(EncryptionLevel level);
   EncryptionLevel encryption_level() { return packet_.encryption_level; }
 
   // packet number of the last created packet, or 0 if no packets have been