Add a histogram to measure the frequency of OCSP stapling support by servers

net/socket/ssl_client_socket_nss.cc

Index: net/socket/ssl_client_socket_nss.cc
diff --git a/net/socket/ssl_client_socket_nss.cc b/net/socket/ssl_client_socket_nss.cc
index d2c9959c389cf09a5bd8e5acdbff874c934fdf0d..598bb7c154bd02dbe32105ba4098c1034fed2957 100644
--- a/net/socket/ssl_client_socket_nss.cc
+++ b/net/socket/ssl_client_socket_nss.cc
@@ -2393,9 +2393,14 @@ void SSLClientSocketNSS::Core::UpdateSignedCertTimestamps() {
 }
 
 void SSLClientSocketNSS::Core::UpdateStapledOCSPResponse() {
+  PRBool ocsp_requested = PR_FALSE;
+  SSL_OptionGet(nss_fd_, SSL_ENABLE_OCSP_STAPLING, &ocsp_requested);

[wtc, 2014/02/19 23:37:02]

Nit: some tool may warn that we ignore the return value. We can say:

    PRBool ocsp_requested;
    if (SSL_OptionGet(nss_fd_, SSL_ENABLE_OCSP_STAPLING, &ocsp_requested) !=
SECSuccess)
      ocsp_requested = PR_FALSE;
  

   const SECItemArray* ocsp_responses =
       SSL_PeerStapledOCSPResponses(nss_fd_);
-  if (!ocsp_responses || !ocsp_responses->len)
+  bool ocsp_responses_present = ocsp_responses && ocsp_responses->len;
+  if (ocsp_requested)
+    UMA_HISTOGRAM_BOOLEAN("Net.OCSPResponseStapled", ocsp_responses_present);
+  if (!ocsp_responses_present)
     return;
 
   nss_handshake_state_.stapled_ocsp_response = std::string(