[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..a7f02c68b8c59820bd7ff73249f62e9345674c46 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="BooleanCacheHit">
+  <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>
@@ -54339,6 +54350,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Notification click (within last 5 seconds)"/>
 </enum>
 
+<enum name="BooleanCacheHit" type="int">
+  <int value="0" label="Miss"/>
+  <int value="1" label="Hit"/>
+</enum>
+
 <enum name="BooleanCanCheckUrl" type="int">
   <int value="0" label="Skipped"/>
   <int value="1" label="Can check"/>