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

ios/web/web_state/ui/crw_wk_web_view_web_controller.mm

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/web/web_state/ui/crw_wk_web_view_web_controller.h"
 
 #import <WebKit/WebKit.h>
 
 #include "base/containers/mru_cache.h"
 #include "base/ios/ios_util.h"
@@ skipping 944 matching lines @@
   scoped_refptr<net::X509Certificate> leafCert;
   BOOL recoverable = NO;
   if (chain.count && host.length) {
     // The complete cert chain may not be available, so the leaf cert is used
     // as a key to retrieve _certVerificationErrors, as well as for storing the
     // cert decision.
     leafCert = web::CreateCertFromChain(@[ chain.firstObject ]);
     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);
     }
   }
 
   // Present SSL interstitial and inform everyone that the load is cancelled.
   [self.delegate presentSSLError:info
                     forSSLStatus:status
                      recoverable:recoverable
                         callback:^(BOOL proceed) {
                           if (proceed) {
                             // The interstitial will be removed during reload.
@@ skipping 837 matching lines @@
         runJavaScriptTextInputPanelWithPrompt:prompt
                                   defaultText:defaultText
                                    requestURL:requestURL
                             completionHandler:completionHandler];
   } else if (completionHandler) {
     completionHandler(nil);
   }
 }
 
 @end