[ios] Make sure that |didFinishNavigation| is signalled for fast goBack.

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 1636 matching lines @@
   [self updateSSLStatusForCurrentNavigationItem];
 }
 
 #endif  // !defined(ENABLE_CHROME_NET_STACK_FOR_WKWEBVIEW)
 
 - (void)webViewLoadingStateDidChange {
   if ([_wkWebView isLoading]) {
     [self addActivityIndicatorTask];
   } else {
     [self clearActivityIndicatorTasks];
+    if ([self currentBackForwardListItemHolder]->navigation_type() ==
+        WKNavigationTypeBackForward) {
+      // A fast back/forward may not call |webView:didFinishNavigation:|, so
+      // finishing the navigation should be signalled explicitly.
+      [self didFinishNavigation];
+    }
   }
 }
 
 - (void)webViewTitleDidChange {
   // WKWebView's title becomes empty when the web process dies; ignore that
   // update.
   if (_webProcessIsDead) {
     DCHECK_EQ(self.title.length, 0U);
     return;
   }
@@ skipping 456 matching lines @@
         runJavaScriptTextInputPanelWithPrompt:prompt
                                   defaultText:defaultText
                                    requestURL:requestURL
                             completionHandler:completionHandler];
   } else if (completionHandler) {
     completionHandler(nil);
   }
 }
 
 @end