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..a56138099ed2b252d094532bc742e2c771f3e9f1 100644
--- a/net/socket/next_proto.h
+++ b/net/socket/next_proto.h
@@ -32,6 +32,14 @@ enum NextProto {
   kProtoMaximumVersion = kProtoQUIC1SPDY3,
 };
 
+// NextProto is contiguous, therefore values can change when protocols are
+// removed.  This function returns invariant values, so that UMA histograms can
+// be combined for different releases.
+unsigned int SpdyProtoPermanentValue(NextProto protocol);
+
+// One larger than the maximum value SpdyProtoPermanentValue can return.
+const unsigned int spdyProtoPermanentValueBoundary = 5;

[Alexei Svitkine (slow), 2014/09/11 18:29:58]

I suggest just making an enum for the PermanentValue version of these values,
then this can be part of the enum and the return value of the function can also
be the enum, so we get type safety (and no chance for someone to accidentally
add a new value but forget to update this).

[Bence, 2014/09/12 12:37:07]

You're right, we need a proper enum here.

This is getting very complicated.  I would in fact prefer to make the values in
NextProto permanent, eliminating the need for an extra enum.    NextProto being
noncontiguous only breaks a single for cycle:
https://code.google.com/p/chromium/codesearch#chromium/src/net/spdy/spdy_test....
 I would prefer to replace that by spelling out every protocol, like in
https://code.google.com/p/chromium/codesearch#chromium/src/net/socket/next_pr....
 This would also eliminate the need for a translating function, so overall
maintenance would be easier.

[Alexei Svitkine (slow), 2014/09/12 14:38:08]

That SGTM.

[Bence, 2014/09/12 20:16:56]

Done.

+
 // List of protocols to use for NPN, used for configuring HttpNetworkSessions.
 typedef std::vector<NextProto> NextProtoVector;