[ios] Log UMA if didStartProvisionalNavigation doesn't add pending item.

ios/web/web_state/ui/crw_web_controller.mm

 // Copyright 2012 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_web_controller.h"
 
 #import <WebKit/WebKit.h>
 
 #import <objc/runtime.h>
 #include <stddef.h>
@@ skipping 5113 matching lines @@
       if (web::GetWebClient()->IsAppSpecificURL(_documentURL)) {
         [self abortLoad];
         NavigationManager::WebLoadParams params(webViewURL);
         [self loadWithParams:params];
       }
       return;
     } else {
       [self registerLoadRequest:webViewURL];
     }
   }
+
+  if (![self currentSessionEntry]) {
+    // In this state CRWWebController will crash in |didCommitNavigation:|
+    // (crbug.com/676458). It's unclear if web controller could get into this
+    // state but it's one of the guesses for crbug.com/676458 root cause. Report
+    // UMA historgam if that happens.
+    // TODO(crbug.com/677552): Remove this historgam.
+    UMA_HISTOGRAM_BOOLEAN(
+        "WebController."
+        "StartProvisionalNavigationExitedWithEmptyNavigationManager",
+        true);
+  }
+
   // Ensure the URL is registered and loadPhase is as expected.
   DCHECK(_lastRegisteredRequestURL == webViewURL);
   DCHECK(self.loadPhase == web::LOAD_REQUESTED);
   _latestWKNavigation.reset([navigation retain]);
 }
 
 - (void)webView:(WKWebView*)webView
     didReceiveServerRedirectForProvisionalNavigation:(WKNavigation*)navigation {
   [self registerLoadRequest:net::GURLWithNSURL(webView.URL)
                    referrer:[self currentReferrer]
@@ skipping 583 matching lines @@
   }
 
   return web::WEB_VIEW_DOCUMENT_TYPE_GENERIC;
 }
 
 - (NSString*)refererFromNavigationAction:(WKNavigationAction*)action {
   return [action.request valueForHTTPHeaderField:@"Referer"];
 }
 
 @end