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

ios/web/web_state/ui/crw_wk_web_view_web_controller.mm

Index: ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
diff --git a/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm b/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
index e901ad813c785614371df2565dcdafbe6ef0abd7..d85fdb7dd346176d1ae3716cea5c257669af5fb2 100644
--- a/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
+++ b/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
@@ -962,12 +962,13 @@ WKWebViewErrorSource WKWebViewErrorSourceFromError(NSError* error) {
     if (leafCert) {
       auto error = _certVerificationErrors->Get(
           {leafCert, base::SysNSStringToUTF8(host)});
-      if (error != _certVerificationErrors->end()) {
+      bool cacheHit = error != _certVerificationErrors->end();
+      if (cacheHit) {
         status.cert_status = error->second.status;
         recoverable = error->second.is_recoverable;
-      } else {
-        // TODO(eugenebut): Report UMA with cache size (crbug.com/541736).
       }
+      UMA_HISTOGRAM_BOOLEAN("WebController.CertVerificationErrorsCacheHit",
+                            cacheHit);
     }
   }