Add const versions of some toHTMLFooElement().

Source/core/html/HTMLMetaElement.h

Index: Source/core/html/HTMLMetaElement.h
diff --git a/Source/core/html/HTMLMetaElement.h b/Source/core/html/HTMLMetaElement.h
index d61fc060efd0a3376ace1cb27b304334b179b59a..4d2db881fd638d9bcf3a4b40219680eaaff16bf9 100644
--- a/Source/core/html/HTMLMetaElement.h
+++ b/Source/core/html/HTMLMetaElement.h
@@ -51,6 +51,12 @@ inline HTMLMetaElement* toHTMLMetaElement(Node* node)
     return static_cast<HTMLMetaElement*>(node);
 }
 
+inline const HTMLMetaElement* toHTMLMetaElement(const Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::metaTag));
+    return static_cast<const HTMLMetaElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif