Remove BackForwardClient, cleanup starting a history navigation

Source/core/page/Page.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All Rights Reserved.
  * Copyright (C) 2008 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 105 matching lines @@
     , m_autoscrollController(AutoscrollController::create(*this))
     , m_chrome(Chrome::create(this, pageClients.chromeClient))
     , m_dragCaretController(DragCaretController::create())
     , m_dragController(DragController::create(this, pageClients.dragClient))
     , m_focusController(FocusController::create(this))
     , m_contextMenuController(ContextMenuController::create(this, pageClients.contextMenuClient))
     , m_inspectorController(InspectorController::create(this, pageClients.inspectorClient))
     , m_pointerLockController(PointerLockController::create(this))
     , m_undoStack(UndoStack::create())
     , m_mainFrame(nullptr)
-    , m_backForwardClient(pageClients.backForwardClient)
     , m_editorClient(pageClients.editorClient)
     , m_spellCheckerClient(pageClients.spellCheckerClient)
     , m_storageClient(pageClients.storageClient)
     , m_openedByDOM(false)
     , m_tabKeyCyclesThroughElements(true)
     , m_defersLoading(false)
     , m_deviceScaleFactor(1)
     , m_timerAlignmentInterval(DOMTimer::visiblePageAlignmentInterval())
     , m_visibilityState(PageVisibilityStateVisible)
     , m_isCursorVisible(true)
@@ skipping 498 matching lines @@
         m_validationMessageClient->willBeDestroyed();
     m_mainFrame = nullptr;
 }
 
 Page::PageClients::PageClients()
     : chromeClient(nullptr)
     , contextMenuClient(nullptr)
     , editorClient(nullptr)
     , dragClient(nullptr)
     , inspectorClient(nullptr)
-    , backForwardClient(nullptr)
     , spellCheckerClient(nullptr)
     , storageClient(nullptr)
 {
 }
 
 Page::PageClients::~PageClients()
 {
 }
 
 } // namespace blink