[ios] Removed canGoBack/canGoForward API from CRWSessionController.

ios/web/navigation/navigation_manager_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/navigation/navigation_manager_impl.h"
 
 #include <stddef.h>
 
 #include <utility>
 
@@ skipping 269 matching lines @@
   NSUInteger idx = static_cast<NSUInteger>(index);
   NSArray* entries = [session_controller_ entries];
   if (idx >= entries.count)
     return false;
 
   [session_controller_ removeEntryAtIndex:index];
   return true;
 }
 
 bool NavigationManagerImpl::CanGoBack() const {
-  return [session_controller_ canGoBack];
+  return CanGoToOffset(-1);
 }
 
 bool NavigationManagerImpl::CanGoForward() const {
-  return [session_controller_ canGoForward];
+  return CanGoToOffset(1);
+}
+
+bool NavigationManagerImpl::CanGoToOffset(int offset) const {
+  return [session_controller_ canGoDelta:offset];
 }
 
 void NavigationManagerImpl::GoBack() {
   delegate_->GoToOffset(-1);
 }
 
 void NavigationManagerImpl::GoForward() {
   delegate_->GoToOffset(1);
 }
 
@@ skipping 22 matching lines @@
 void NavigationManagerImpl::RemoveTransientURLRewriters() {
   transient_url_rewriters_.reset();
 }
 
 void NavigationManagerImpl::CopyState(
     NavigationManagerImpl* navigation_manager) {
   SetSessionController([navigation_manager->GetSessionController() copy]);
 }
 
 }  // namespace web