Add toRenderSVGResourceMasker() fuction, and use it.

Source/core/rendering/svg/RenderSVGResourceMasker.h

Index: Source/core/rendering/svg/RenderSVGResourceMasker.h
diff --git a/Source/core/rendering/svg/RenderSVGResourceMasker.h b/Source/core/rendering/svg/RenderSVGResourceMasker.h
index f834bee388ffded7dd0945e437a9845c25601e23..309ec6ffbdbf0f9bb518afbea42f32086dab5c36 100644
--- a/Source/core/rendering/svg/RenderSVGResourceMasker.h
+++ b/Source/core/rendering/svg/RenderSVGResourceMasker.h
@@ -59,6 +59,12 @@ private:
     FloatRect m_maskContentBoundaries;
 };
 
+inline RenderSVGResourceMasker* toRenderSVGResourceMasker(RenderSVGResource* resource)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!resource || resource->resourceType() == MaskerResourceType);
+    return static_cast<RenderSVGResourceMasker*>(resource);
+}
+
 }
 
 #endif