Introduce new SPDY Version UMA histogram.

net/socket/next_proto.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_SOCKET_NEXT_PROTO_H_
 #define NET_SOCKET_NEXT_PROTO_H_
 
 #include <vector>
 
 #include "net/base/net_export.h"
 
 namespace net {
 
 // Next Protocol Negotiation (NPN), if successful, results in agreement on an
 // application-level string that specifies the application level protocol to
 // use over the TLS connection. NextProto enumerates the application level
-// protocols that we recognise.
+// protocols that we recognise.  Do not change or reuse values, because they
+// are used to collect statistics on UMA.
 enum NextProto {
   kProtoUnknown = 0,
-  kProtoHTTP11,
+  kProtoHTTP11 = 1,
   kProtoMinimumVersion = kProtoHTTP11,
 
-  kProtoDeprecatedSPDY2,
+  kProtoDeprecatedSPDY2 = 100,
   kProtoSPDYMinimumVersion = kProtoDeprecatedSPDY2,
-  kProtoSPDY3,
-  kProtoSPDY31,
-  kProtoSPDY4,  // SPDY4 is HTTP/2.
+  kProtoSPDY3 = 101,
+  kProtoSPDY31 = 102,
+  kProtoSPDY4 = 103,  // SPDY4 is HTTP/2.
   kProtoSPDYMaximumVersion = kProtoSPDY4,
 
-  kProtoQUIC1SPDY3,
+  kProtoQUIC1SPDY3 = 200,
 
   kProtoMaximumVersion = kProtoQUIC1SPDY3,
 };
 
 // List of protocols to use for NPN, used for configuring HttpNetworkSessions.
 typedef std::vector<NextProto> NextProtoVector;
 
 // Convenience functions to create NextProtoVector.
 
 NET_EXPORT NextProtoVector NextProtosHttpOnly();
 
 // Default values, which are subject to change over time.  Currently just
 // SPDY 3 and 3.1.
 NET_EXPORT NextProtoVector NextProtosDefaults();
 
 NET_EXPORT NextProtoVector NextProtosWithSpdyAndQuic(bool spdy_enabled,
                                                      bool quic_enabled);
 
 // All of these also enable QUIC.
 NET_EXPORT NextProtoVector NextProtosSpdy3();
 NET_EXPORT NextProtoVector NextProtosSpdy31();
 NET_EXPORT NextProtoVector NextProtosSpdy31WithSpdy2();
 NET_EXPORT NextProtoVector NextProtosSpdy4Http2();
 
 }  // namespace net
 
 #endif  // NET_SOCKET_NEXT_PROTO_H_