Remove BackForwardClient, cleanup starting a history navigation

Source/web/FrameLoaderClientImpl.h

 /*
  * Copyright (C) 2009, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 152 matching lines @@
     virtual PassOwnPtr<WebServiceWorkerProvider> createServiceWorkerProvider() override;
     virtual bool isControlledByServiceWorker(DocumentLoader&) override;
     virtual SharedWorkerRepositoryClient* sharedWorkerRepositoryClient() override;
 
     virtual PassOwnPtr<WebApplicationCacheHost> createApplicationCacheHost(WebApplicationCacheHostClient*) override;
 
     virtual void didStopAllLoaders() override;
 
     virtual void dispatchDidChangeManifest() override;
 
+    virtual unsigned backForwardLength() override;
+
 private:
     virtual bool isFrameLoaderClientImpl() const override { return true; }
 
     PassOwnPtr<WebPluginLoadObserver> pluginLoadObserver(DocumentLoader*);
 
     // The WebFrame that owns this object and manages its lifetime. Therefore,
     // the web frame object is guaranteed to exist.
     WebLocalFrameImpl* m_webFrame;
 };
 
 DEFINE_TYPE_CASTS(FrameLoaderClientImpl, FrameLoaderClient, client, client->isFrameLoaderClientImpl(), client.isFrameLoaderClientImpl());
 
 } // namespace blink
 
 #endif