Add metrics for certificate report uploads

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 64cee9e5b661709954d062fceff40ce4c22831f0..8dce6f60b2e0fb56aa097c126782e58289a93398 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -69396,6 +69396,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="SSL.CertificateErrorReportEvent"

[Jialiu Lin, 2017/05/05 00:25:43]

nit: SSL.CertificateErrorReportOutcome?

[meacer, 2017/05/05 19:24:20]

Done, this and the enum.

+    enum="CertificateReportEventCode">
+  <owner>meacer@chromium.org</owner>
+  <summary>
+    Users can opt in to send reports of certificate validation errors to Google.
+    This records the event code whenever Chrome sends such a report.
+  </summary>
+</histogram>
+
 <histogram name="SSL.CertificateErrorReportFailure" enum="NetErrorCodes">
   <owner>estark@chromium.org</owner>
   <summary>
@@ -86272,6 +86281,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="0" label="Root Certificate"/>
 </enum>
 
+<enum name="CertificateReportEventCode" type="int">
+  <int value="0" label="REPORT_SUBMITTED"/>
+  <int value="1" label="REPORT_FAILED"/>
+  <int value="2" label="REPORT_SUCCESSFUL"/>
+  <int value="3" label="REPORT_DROPPED_OR_IGNORED"/>
+</enum>
+
 <enum name="CertificateTransparencyDnsQueryStatus" type="int">
   <int value="0" label="SUCCESS"/>
   <int value="1" label="FAILED_UNKNOWN"/>