Introduce new SPDY Version UMA histogram.

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 19ff55e0bc020fe3b9e3c213c1c6f5af9f985167..08e6801532791ceea88365a9a6121b911db19433 100644
--- a/net/socket/next_proto.h
+++ b/net/socket/next_proto.h
@@ -14,20 +14,21 @@ 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,
 };