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

Source/core/accessibility/AccessibilityRenderObject.cpp

Index: Source/core/accessibility/AccessibilityRenderObject.cpp
diff --git a/Source/core/accessibility/AccessibilityRenderObject.cpp b/Source/core/accessibility/AccessibilityRenderObject.cpp
index f1d0f904c3a99f3356ead9f1cc59d24e9157b118..6157916255cdf4a23947dfbc00616e9a1e8aa80b 100644
--- a/Source/core/accessibility/AccessibilityRenderObject.cpp
+++ b/Source/core/accessibility/AccessibilityRenderObject.cpp
@@ -2086,8 +2086,7 @@ AccessibilitySVGRoot* AccessibilityRenderObject::remoteSVGRootElement() const
     if (!image || !image->isSVGImage())
         return 0;
 
-    SVGImage* svgImage = static_cast<SVGImage*>(image);
-    FrameView* frameView = svgImage->frameView();
+    FrameView* frameView = toSVGImage(image)->frameView();
     if (!frameView)
         return 0;
     Document* doc = frameView->frame().document();