Add toSVGImage fuction, and use it as much as we can.

Source/core/svg/graphics/SVGImage.h

 /*
  * Copyright (C) 2006 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2009 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 84 matching lines @@
     virtual void draw(GraphicsContext*, const FloatRect& fromRect, const FloatRect& toRect, CompositeOperator, BlendMode) OVERRIDE;
     void drawForContainer(GraphicsContext*, const FloatSize, float, const FloatRect&, const FloatRect&, CompositeOperator, BlendMode);
     void drawPatternForContainer(GraphicsContext*, const FloatSize, float, const FloatRect&, const FloatSize&, const FloatPoint&,
         CompositeOperator, const FloatRect&, BlendMode);
 
     OwnPtr<SVGImageChromeClient> m_chromeClient;
     OwnPtr<Page> m_page;
     IntSize m_intrinsicSize;
 };
 
+inline SVGImage* toSVGImage(Image* image)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!image || image->isSVGImage());
+    return static_cast<SVGImage*>(image);
+}
+
 class ImageObserverDisabler {
     WTF_MAKE_NONCOPYABLE(ImageObserverDisabler);
 public:
     ImageObserverDisabler(Image* image)
         : m_image(image)
     {
         ASSERT(m_image->imageObserver());
         m_observer = m_image->imageObserver();
         m_image->setImageObserver(0);
     }
 
     ~ImageObserverDisabler()
     {
         m_image->setImageObserver(m_observer);
     }
 private:
     Image* m_image;
     ImageObserver* m_observer;
 };
 
 }
 
 #endif // SVGImage_h