Replace SSLProtocolNegotiation histogram with SSLNegotiatedAlpnProtocol.

net/socket/next_proto.h

Index: net/socket/next_proto.h
diff --git a/net/socket/next_proto.h b/net/socket/next_proto.h
index 8f90b44b5547b8036e6709dbab1b7c9a6e1b4166..25f0277bb581256cc69655139c3c771a3753812b 100644
--- a/net/socket/next_proto.h
+++ b/net/socket/next_proto.h
@@ -11,24 +11,14 @@
 
 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 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.
-const int kProtoSPDYHistogramOffset = 100;
+// This enum is used in Net.SSLNegotiatedAlpnProtocol histogram.
+// Do not change or re-use values.
 enum NextProto {
   kProtoUnknown = 0,
   kProtoHTTP11 = 1,
-  // kProtoSPDY31 = 102,
-  // kProtoHTTP2_14 = 103,  // HTTP/2 draft-14
-  // kProtoHTTP2_15 = 104,  // HTTP/2 draft-15
-  // kProtoHTTP2_16 = 105,  // HTTP/2 draft-16
-  // kProtoHTTP2_17 = 106,  // HTTP/2 draft-17
-  kProtoHTTP2 = 107,  // HTTP/2, see https://tools.ietf.org/html/rfc7540.
-  kProtoQUIC1SPDY3 = 200,
+  kProtoHTTP2 = 2,
+  kProtoQUIC1SPDY3 = 3,
+  kProtoLast = kProtoQUIC1SPDY3

[jwd, 2016/08/29 19:44:20]

I'd prefer it if you kProtoLast was the boundary value, rather than doing the +
1 later. You can rename it to count or max or something, if you wish.

[davidben, 2016/08/29 19:47:33]

The nice thing about the boundary value is we don't need to stick a dummy case
in every enum switch-case.

'count' is confusing if there are holes. 'max' is also a bit ambiguous as to
whether it should be max(all_next_proto_values) or just some number such
next_proto < max for all next_proto.

[Bence, 2016/08/30 00:51:06]

I agree with davidben@.  See Patch Set 7 for all the places that a dummy value
needs to be handled in every switch() to avoid compiler errors.

[jwd, 2016/09/14 18:24:00]

Alright, that's fine.

 };
 
 // List of protocols to use for NPN, used for configuring HttpNetworkSessions.