Add --ignore-certificate-errors-spki-list switch and UMA histogram.

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 a9161fccf87273ba6849d47cc82dfe9844199e0b..87c16a8c3dc244a1ecbcd932f2f331e874e5a786 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -31679,6 +31679,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.Certificate.IgnoreCertificateErrorsSPKIList"
+    enum="BooleanEnabled">

[Ilya Sherman, 2017/04/26 00:14:33]

This enum has roughly opposite labels to what's being recorded, IMO.  I'd
recommend (just in this file) using an enum with more specific labels, like
"Validation enabled" and "Validation disabled for some keys".

[martinkr, 2017/04/26 00:57:03]

How about adding "Present" to emphasize that this is recording flag presence and
changing the enum accordingly?

+  <owner>martinkr@google.com</owner>
+  <summary>
+    Whether certificate validation was disabled for a set of public keys via the
+    --ignore-certificate-errors-spki-list flag.

[Ilya Sherman, 2017/04/26 00:14:33]

Please document when this is recorded.

[martinkr, 2017/04/26 00:57:03]

Done.

+  </summary>
+</histogram>
+
 <histogram name="Net.Certificate.IgnoreErrors" enum="BooleanEnabled">
   <owner>martinkr@google.com</owner>
   <summary>