Cleanup: Add more conversion helpers for usage.

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 145 matching lines @@
     virtual void didStopAllLoaders() OVERRIDE;
 
 private:
     PassOwnPtr<WebPluginLoadObserver> pluginLoadObserver();
 
     // The WebFrame that owns this object and manages its lifetime. Therefore,
     // the web frame object is guaranteed to exist.
     WebFrameImpl* m_webFrame;
 };
 
+inline FrameLoaderClientImpl* toFrameLoaderClientImpl(WebCore::FrameLoaderClient* client)
+{
+    // We need not check for type of FrameLoaderClient since FrameLoaderClientImpl
+    // is the only derived class of FrameLoaderClient.
+    ASSERT_WITH_SECURITY_IMPLICATION(!client);

[tkent, 2013/10/08 05:24:04]

Null pointer has no security implication.
Please do not reject null client for consistency.

+    return static_cast<FrameLoaderClientImpl*>(client);
+}
+
 } // namespace WebKit
 
 #endif