Refactoring: Add toWebFrameImpl() interface.

Source/web/tests/FrameLoaderClientImplTest.cpp

 /*
  * Copyright (C) 2011, 2012 Google 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.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 52 matching lines @@
 private:
     WebString m_userAgentOverride;
 };
 
 class FrameLoaderClientImplTest : public testing::Test {
 public:
     void SetUp()
     {
         m_webView = WebView::create(0);
         m_webView->initializeMainFrame(&m_webFrameClient);
-        WebFrameImpl* frame = static_cast<WebFrameImpl*>(m_webView->mainFrame());
-        m_frameLoaderClientImpl = static_cast<FrameLoaderClientImpl*>(frame->frame()->loader()->client());
+        m_frameLoaderClientImpl = static_cast<FrameLoaderClientImpl*>(toWebFrameImpl(m_webView->mainFrame())->frame()->loader()->client());
     }
 
     void TearDown()
     {
         m_webView->close();
     }
 
     void setUserAgentOverride(const WebString& userAgent)
     {
         return m_webFrameClient.setUserAgentOverride(userAgent);
@@ skipping 21 matching lines @@
     // Override the user agent and make sure we get it back.
     setUserAgentOverride(override);
     EXPECT_TRUE(override.equals(userAgent()));
 
     // Remove the override and make sure we get the original back.
     setUserAgentOverride(WebString());
     EXPECT_TRUE(defaultUserAgent.equals(userAgent()));
 }
 
 } // namespace