Add histogram to track NPN/ALPN.

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.  Do not change or reuse values, because they
-// are used to collect statistics on UMA.
+// protocols that we recognize.  Do not change or reuse values, because they
+// are used to collect statistics on UMA.  Also, values must be in [0,499),
+// because of the way TLS protocol negotiation extension information is added to
+// UMA histogram.
 enum NextProto {
   kProtoUnknown = 0,
   kProtoHTTP11 = 1,
   kProtoMinimumVersion = kProtoHTTP11,
 
   kProtoDeprecatedSPDY2 = 100,
   kProtoSPDYMinimumVersion = kProtoDeprecatedSPDY2,
   kProtoSPDY3 = 101,
   kProtoSPDY31 = 102,
   kProtoSPDY4 = 103,  // SPDY4 is HTTP/2.
@@ skipping 20 matching lines @@
 
 // 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_