[ios] UMA metric to report cache miss for WKWebView cert verification.

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 42330e5e123a21c856080f82647b4c8755e620c8..9056c17752e72c6bb97b3fb6b8152b0d170426ca 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -50611,6 +50611,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="WebController.CertVerificationErrorsCacheHit"
+    enum="BooleanSuccess">

[Ilya Sherman, 2015/11/05 21:31:13]

Please define an enum with custom labels for this histogram.

[Eugene But (OOO till 7-30), 2015/11/05 21:35:07]

What is the advantage of a custom enum? "BooleanSuccess" is used by many
diagrams and seems to be suitable for this one.

[Ilya Sherman, 2015/11/05 21:42:01]

"Hit" vs. "Miss" is much clearer than "Success" vs "Failure" for a histogram
measuring cache hits.  Note that this is purely a histograms.xml metadata
change; no C++/Obj-C changes needed.

[Eugene But (OOO till 7-30), 2015/11/05 21:55:30]

Sure. Done.

+  <owner>eugenebut@chromium.org</owner>
+  <summary>
+    [iOS] Report cache hit/miss for WKWebView cert verification. WKWebView Web
+    Controller has a cache of pending cert verification results to avoid extra
+    verifications when presenting SSL interstitial. This metric helps to
+    understand whether or not cache miss is possible.
+  </summary>
+</histogram>
+
 <histogram name="WebController.ExternalURLRequestBlocking"
     enum="IOSExternalURLRequestStatus">
   <owner>jyquinn@chromium.org</owner>