Replace SSLProtocolNegotiation histogram with SSLNegotiatedAlpnProtocol.

* Replace sparse histogram recording ALPN/NPN usage and whether there was
  overlap or fallback in case of NPN with a new histogram with much fewer
  values.  This was made possible by NPN being disabled.
* New histogram is recorded with all TLS handshakes, telling us what
  percentage of total TLS connections use HTTP/2.  (The old histogram was only
  recorded when either NPN or ALPN was used, depriving us of this piece of
  information.)
* Remove fixed values from enum NextProto; remove commented out historical
  values.  Note that git cl format puts the new enum on a single line.

BUG=526713

Review-Url: https://codereview.chromium.org/2243453002
Cr-Commit-Position: refs/heads/master@{#418664}

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

net/socket/ssl_client_socket_impl.cc

@@ -1219,7 +1219,7 @@ int SSLClientSocketImpl::DoHandshakeComplete(int result) {
     }
   }
 
-  RecordNegotiationExtension();
+  RecordNegotiatedProtocol();
   RecordChannelIDSupport();
 
   const uint8_t* ocsp_response_raw;
@@ -2337,26 +2337,9 @@ void SSLClientSocketImpl::LogConnectEndEvent(int rv) {
                     base::Bind(&NetLogSSLInfoCallback, base::Unretained(this)));
 }
 
-void SSLClientSocketImpl::RecordNegotiationExtension() const {
-  if (negotiation_extension_ == kExtensionUnknown)
-    return;
-  if (npn_status_ == kNextProtoUnsupported)
-    return;
-  base::HistogramBase::Sample sample =
-      static_cast<base::HistogramBase::Sample>(negotiated_protocol_);
-  // In addition to the protocol negotiated, we want to record which TLS
-  // extension was used, and in case of NPN, whether there was overlap between
-  // server and client list of supported protocols.
-  if (negotiation_extension_ == kExtensionNPN) {
-    if (npn_status_ == kNextProtoNoOverlap) {
-      sample += 1000;
-    } else {
-      sample += 500;
-    }
-  } else {
-    DCHECK_EQ(kExtensionALPN, negotiation_extension_);
-  }
-  UMA_HISTOGRAM_SPARSE_SLOWLY("Net.SSLProtocolNegotiation", sample);
+void SSLClientSocketImpl::RecordNegotiatedProtocol() const {
+  UMA_HISTOGRAM_ENUMERATION("Net.SSLNegotiatedAlpnProtocol",
+                            negotiated_protocol_, kProtoLast + 1);
 }
 
 void SSLClientSocketImpl::RecordChannelIDSupport() const {

net/socket/ssl_client_socket_impl.h

@@ -240,9 +240,9 @@ class SSLClientSocketImpl : public SSLClientSocket {
 
   void LogConnectEndEvent(int rv);
 
-  // Record which TLS extension was used to negotiate protocol and protocol
-  // chosen in a UMA histogram.
-  void RecordNegotiationExtension() const;
+  // Record whether ALPN was used, and if so, the negotiated protocol,
+  // in a UMA histogram.
+  void RecordNegotiatedProtocol() const;
 
   // Records histograms for channel id support during full handshakes - resumed
   // handshakes are ignored.

tools/metrics/histograms/histograms.xml

@@ -32873,6 +32873,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Time to complete a speculative certificate verification.</summary>
 </histogram>
 
+<histogram name="Net.SSLNegotiatedAlpnProtocol"
+    enum="SSLNegotiatedAlpnProtocol">
+  <owner>bnc@chromium.org</owner>
+  <summary>
+    For each TLS handshake, whether ALPN was negotiated; and if so, the
+    negotiated protocol.
+  </summary>
+</histogram>
+
 <histogram name="Net.SSLProtocolErrorCipher" enum="SSLCipherSuite">
   <obsolete>
     Removed in March 2016.
@@ -32899,6 +32908,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="Net.SSLProtocolNegotiation" enum="SSLProtocolNegotiation">
+  <obsolete>
+    Superseded by Net.SSLNegotiatedAlpnProtocol in 2016 August.
+  </obsolete>
   <owner>bnc@chromium.org</owner>
   <summary>
     TLS extension used to negotiate protocol (ALPN or NPN); in case of NPN,
@@ -96408,6 +96420,12 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="3" label="NOT_EXPIRED_AND_DO_NOT_PROCEED"/>
 </enum>
 
+<enum name="SSLNegotiatedAlpnProtocol" type="int">
+  <int value="0" label="ALPN not used"/>
+  <int value="1" label="HTTP/1.1 negotiated via ALPN"/>
+  <int value="2" label="HTTP/2 negotiated via ALPN"/>
+</enum>
+
 <enum name="SSLOrQUICVersion" type="int">
   <int value="0" label="Unknown"/>
   <int value="1" label="SSL 2.0"/>