Add a histogram for measuring the number of times we fall back to common name matching, when a cert…

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 5dfbe96144e13d334d29af148d37a16f09459d49..6c61f8af495c547344012da1a7e0cd27e389e0af 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -6725,6 +6725,14 @@ other types of suffix sets.
   <summary>The time to generate an NTLM HTTP authentication token.</summary>
 </histogram>
 
+<histogram name="Net.CertCommonNameFallback" enum="BooleanSuccess">
+  <summary>
+    Whether certificate hostname matching had to fall back to using the

[jar (doing other things), 2013/10/17 16:44:45]

nit: I'm not sure what "success" or "failed" would mean... which case it would
stand  for.

You should probably either illuminate that in the prose here (putting the words
in quotes for teh given case.... or probably better... declare you own enum with
tags like "fallback to Common Name"  vs "use subjectAltName" 

+    certificate's Common Name because it lacked the subjectAltName extension or
+    lacked any DNS or IP subjectAltNames.
+  </summary>
+</histogram>
+
 <histogram name="Net.CertificatePinSuccess" enum="BooleanSuccess">
   <obsolete>
     Renamed to Net.PublicKeyPinSuccess 28 Oct 2011.