Adds UMA for slow and fast back/forward WKWebController navigations.

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 <stddef.h>
 
 #include <utility>
@@ skipping 723 matching lines @@
   [super handleLoadError:error inMainFrame:inMainFrame];
 }
 
 // Override |loadCancelled| to |cancelPendingDownload| for the
 // CRWPassKitDownloader.
 - (void)loadCancelled {
   [_passKitDownloader cancelPendingDownload];
   [super loadCancelled];
 }
 
+- (BOOL)canPerformFastNavigationForSessionEntry:(CRWSessionEntry*)sessionEntry {
+  web::WKBackForwardListItemHolder* holder =
+      web::WKBackForwardListItemHolder::FromNavigationItem(
+          sessionEntry.navigationItemImpl);
+  return (holder->back_forward_list_item() &&
+          [self isBackForwardListItemValid:holder->back_forward_list_item()]);
+}
+
 - (BOOL)isViewAlive {
   return !_webProcessIsDead && [super isViewAlive];
 }
 
 #pragma mark Private methods
 
 - (NSDictionary*)wkWebViewObservers {
   return @{
     @"certificateChain" : @"webViewSecurityFeaturesDidChange",
     @"estimatedProgress" : @"webViewEstimatedProgressDidChange",
@@ skipping 1295 matching lines @@
         runJavaScriptTextInputPanelWithPrompt:prompt
                                   defaultText:defaultText
                                    requestURL:requestURL
                             completionHandler:completionHandler];
   } else if (completionHandler) {
     completionHandler(nil);
   }
 }
 
 @end