Add toHTMLDocument() convenience methods that take a reference

Source/core/html/HTMLDocument.h

Index: Source/core/html/HTMLDocument.h
diff --git a/Source/core/html/HTMLDocument.h b/Source/core/html/HTMLDocument.h
index 6990a78bdba86226742ae5106a4008a3e70f26a3..181b94203088614b3c722ffd7b684110f29fb6b4 100644
--- a/Source/core/html/HTMLDocument.h
+++ b/Source/core/html/HTMLDocument.h
@@ -112,6 +112,18 @@ inline const HTMLDocument* toHTMLDocument(const Document* document)
     return static_cast<const HTMLDocument*>(document);
 }
 
+inline HTMLDocument& toHTMLDocument(Document& document)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(document.isHTMLDocument());
+    return static_cast<HTMLDocument&>(document);
+}
+
+inline const HTMLDocument& toHTMLDocument(const Document& document)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(document.isHTMLDocument());
+    return static_cast<const HTMLDocument&>(document);
+}
+
 // This will catch anyone doing an unnecessary cast.
 void toHTMLDocument(const HTMLDocument*);