Add toHTMLMetaElement(), and use it.

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 5d16b4d85f565b8db37a48a98a3eff4da8a0c634..d61fc060efd0a3376ace1cb27b304334b179b59a 100644
--- a/Source/core/html/HTMLMetaElement.h
+++ b/Source/core/html/HTMLMetaElement.h
@@ -45,6 +45,12 @@ private:
     void process();
 };
 
-} //namespace
+inline HTMLMetaElement* toHTMLMetaElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::metaTag));
+    return static_cast<HTMLMetaElement*>(node);
+}
+
+} // namespace WebCore
 
 #endif