Add toRenderSVGResourceClipper() fuction, and use it.

Source/core/rendering/RenderLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 3556 matching lines @@
             ReferenceClipPathOperation* referenceClipPathOperation = static_cast<ReferenceClipPathOperation*>(style->clipPath());
             Document& document = renderer()->document();
             // FIXME: It doesn't work with forward or external SVG references (https://bugs.webkit.org/show_bug.cgi?id=90405)
             Element* element = document.getElementById(referenceClipPathOperation->fragment());
             if (element && element->hasTagName(SVGNames::clipPathTag) && element->renderer()) {
                 if (!rootRelativeBoundsComputed) {
                     rootRelativeBounds = calculateLayerBounds(paintingInfo.rootLayer, &offsetFromRoot, 0);
                     rootRelativeBoundsComputed = true;
                 }
 
-                // FIXME: This should use a safer cast such as toRenderSVGResourceContainer().
-                resourceClipper = static_cast<RenderSVGResourceClipper*>(element->renderer());
+                resourceClipper = toRenderSVGResourceClipper(element->renderer()->toRenderSVGResourceContainer());
                 if (!resourceClipper->applyClippingToContext(renderer(), rootRelativeBounds, paintingInfo.paintDirtyRect, context)) {
                     // No need to post-apply the clipper if this failed.
                     resourceClipper = 0;
                 }
             }
         }
     }
 
     LayerPaintingInfo localPaintingInfo(paintingInfo);
     FilterEffectRendererHelper filterPainter(filterRenderer() && paintsWithFilters());
@@ skipping 2768 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif