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

Source/core/svg/graphics/SVGImage.h

Index: Source/core/svg/graphics/SVGImage.h
diff --git a/Source/core/svg/graphics/SVGImage.h b/Source/core/svg/graphics/SVGImage.h
index aa5aaebd417556097ea99b0be88295afc6b82951..b75dd70e29e7640c280a63b9b1c680dac7d2ace0 100644
--- a/Source/core/svg/graphics/SVGImage.h
+++ b/Source/core/svg/graphics/SVGImage.h
@@ -102,6 +102,12 @@ private:
     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: