Added const getter for WebState's NavigationManager.

ios/web/web_state/web_state_impl.mm

 // Copyright 2013 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.
 
 #include "ios/web/web_state/web_state_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include "base/strings/sys_string_conversions.h"
@@ skipping 535 matching lines @@
 BrowserState* WebStateImpl::GetBrowserState() const {
   return navigation_manager_.GetBrowserState();
 }
 
 void WebStateImpl::OpenURL(const WebState::OpenURLParams& params) {
   DCHECK(Configured());
   ClearTransientContentView();
   [[web_controller_ delegate] openURLWithParams:params];
 }
 
+const NavigationManager* WebStateImpl::GetNavigationManager() const {
+  return &GetNavigationManagerImpl();
+}
+
 NavigationManager* WebStateImpl::GetNavigationManager() {
   return &GetNavigationManagerImpl();
 }
 
 CRWJSInjectionReceiver* WebStateImpl::GetJSInjectionReceiver() const {
   return [web_controller_ jsInjectionReceiver];
 }
 
 void WebStateImpl::ExecuteJavaScript(const base::string16& javascript) {
   [web_controller_ executeJavaScript:base::SysUTF16ToNSString(javascript)
@@ skipping 93 matching lines @@
     const LoadCommittedDetails& load_details) {
   FOR_EACH_OBSERVER(WebStateObserver, observers_,
                     NavigationItemCommitted(load_details));
 }
 
 WebState* WebStateImpl::GetWebState() {
   return this;
 }
 
 }  // namespace web