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

Source/core/rendering/RenderImage.cpp

Index: Source/core/rendering/RenderImage.cpp
diff --git a/Source/core/rendering/RenderImage.cpp b/Source/core/rendering/RenderImage.cpp
index c1ffbcd103850a5a904b54e3cc1b368b6b461297..e622c9c11317792966e7f71abf6e04e1b55f2bdd 100644
--- a/Source/core/rendering/RenderImage.cpp
+++ b/Source/core/rendering/RenderImage.cpp
@@ -618,7 +618,7 @@ RenderBox* RenderImage::embeddedContentBox() const
 
     ImageResource* cachedImage = m_imageResource->cachedImage();
     if (cachedImage && cachedImage->image() && cachedImage->image()->isSVGImage())
-        return static_cast<SVGImage*>(cachedImage->image())->embeddedContentBox();
+        return toSVGImage(cachedImage->image())->embeddedContentBox();
 
     return 0;
 }