Add comments describing changes introduced in each QUIC version.

net/quic/quic_protocol.h

Index: net/quic/quic_protocol.h
diff --git a/net/quic/quic_protocol.h b/net/quic/quic_protocol.h
index d76e12ae7c11c7ec45006a786a5eb0d58bb1b4da..38254ff564b8e14d1a727ccd57463453bc68e16c 100644
--- a/net/quic/quic_protocol.h
+++ b/net/quic/quic_protocol.h
@@ -278,12 +278,12 @@ enum QuicVersion {
   // Special case to indicate unknown/unsupported QUIC version.
   QUIC_VERSION_UNSUPPORTED = 0,
 
-  QUIC_VERSION_15 = 15,
-  QUIC_VERSION_16 = 16,
-  QUIC_VERSION_18 = 18,
-  QUIC_VERSION_19 = 19,
-  QUIC_VERSION_20 = 20,
-  QUIC_VERSION_21 = 21,  // Current version.
+  QUIC_VERSION_15 = 15,  // Revived packets in ReceivedPacketInfo.
+  QUIC_VERSION_16 = 16,  // STOP_WAITING frame.
+  QUIC_VERSION_18 = 18,  // PING frame.
+  QUIC_VERSION_19 = 19,  // Session level flow control.
+  QUIC_VERSION_20 = 20,  // Independent stream/session flow control windows.
+  QUIC_VERSION_21 = 21,  // Headers/crypto streams are flow controlled.
 };
 
 // This vector contains QUIC versions which we currently support.