Add a dedicated SSL protocol version metric.

net/ssl/ssl_connection_status_flags.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_SSL_SSL_CONNECTION_STATUS_FLAGS_H_
 #define NET_SSL_SSL_CONNECTION_STATUS_FLAGS_H_
 
 #include "base/logging.h"
 #include "base/macros.h"
 
@@ skipping 19 matching lines @@
 
   // The next three bits are reserved for the SSL version.
   SSL_CONNECTION_VERSION_SHIFT = 20,
   SSL_CONNECTION_VERSION_MASK = 7,
 
   // 1 << 31 (the sign bit) is reserved so that the SSL connection status will
   // never be negative.
 };
 
 // NOTE: the SSL version enum constants must be between 0 and
-// SSL_CONNECTION_VERSION_MASK, inclusive.
+// SSL_CONNECTION_VERSION_MASK, inclusive. These values are persisted to disk
+// and used in UMA, so they must remain stable.
 enum {
   SSL_CONNECTION_VERSION_UNKNOWN = 0,  // Unknown SSL version.
   SSL_CONNECTION_VERSION_SSL2 = 1,
   SSL_CONNECTION_VERSION_SSL3 = 2,
   SSL_CONNECTION_VERSION_TLS1 = 3,
   SSL_CONNECTION_VERSION_TLS1_1 = 4,
   SSL_CONNECTION_VERSION_TLS1_2 = 5,
   // Reserve 6 for TLS 1.3.
   SSL_CONNECTION_VERSION_QUIC = 7,
   SSL_CONNECTION_VERSION_MAX,
@@ skipping 26 matching lines @@
   *connection_status &=
       ~(SSL_CONNECTION_VERSION_MASK << SSL_CONNECTION_VERSION_SHIFT);
   // Set the new version.
   *connection_status |=
       ((version & SSL_CONNECTION_VERSION_MASK) << SSL_CONNECTION_VERSION_SHIFT);
 }
 
 }  // namespace net
 
 #endif  // NET_SSL_SSL_CONNECTION_STATUS_FLAGS_H_