Remove BackForwardClient, cleanup starting a history navigation

Source/core/loader/EmptyClients.cpp

 /*
  * Copyright (C) 2006 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 42 matching lines @@
 
     static DragClient* dummyDragClient = adoptPtr(new EmptyDragClient).leakPtr();
     pageClients.dragClient = dummyDragClient;
 
     static EditorClient* dummyEditorClient = adoptPtr(new EmptyEditorClient).leakPtr();
     pageClients.editorClient = dummyEditorClient;
 
     static InspectorClient* dummyInspectorClient = adoptPtr(new EmptyInspectorClient).leakPtr();
     pageClients.inspectorClient = dummyInspectorClient;
 
-    static BackForwardClient* dummyBackForwardClient = adoptPtr(new EmptyBackForwardClient).leakPtr();
-    pageClients.backForwardClient = dummyBackForwardClient;
-
     static SpellCheckerClient* dummySpellCheckerClient = adoptPtr(new EmptySpellCheckerClient).leakPtr();
     pageClients.spellCheckerClient = dummySpellCheckerClient;
 
     static StorageClient* dummyStorageClient = adoptPtr(new EmptyStorageClient).leakPtr();
     pageClients.storageClient = dummyStorageClient;
 }
 
 class EmptyPopupMenu : public PopupMenu {
 public:
     virtual void show(const FloatQuad&, const IntSize&, int) override { }
@@ skipping 85 matching lines @@
 {
     return nullptr;
 }
 
 PassOwnPtr<StorageNamespace> EmptyStorageClient::createSessionStorageNamespace()
 {
     return nullptr;
 }
 
 }