Add a comment about why HTMLObjectElement inherits FormAssociatedElement

third_party/WebKit/Source/core/html/HTMLObjectElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 13 matching lines @@
 #define HTMLObjectElement_h
 
 #include "core/CoreExport.h"
 #include "core/html/FormAssociatedElement.h"
 #include "core/html/HTMLPlugInElement.h"
 
 namespace blink {
 
 class HTMLFormElement;
 
+// Inheritance of FormAssociatedElement was used for NPAPI form association, but is still
+// kept here so that legacy APIs such as form attribute can keep working according to the spec.
+// See: https://html.spec.whatwg.org/multipage/embedded-content.html#the-object-element
 class CORE_EXPORT HTMLObjectElement final : public HTMLPlugInElement, public FormAssociatedElement {
     DEFINE_WRAPPERTYPEINFO();
     USING_GARBAGE_COLLECTED_MIXIN(HTMLObjectElement);
 public:
     static HTMLObjectElement* create(Document&, HTMLFormElement*, bool createdByParser);
     ~HTMLObjectElement() override;
     DECLARE_VIRTUAL_TRACE();
 
     const String& classId() const { return m_classId; }
 
@@ skipping 84 matching lines @@
     const HTMLObjectElement& objectElement = static_cast<const HTMLObjectElement&>(element);
     // We need to assert after the cast because FormAssociatedElement doesn't
     // have hasTagName.
     ASSERT_WITH_SECURITY_IMPLICATION(objectElement.hasTagName(HTMLNames::objectTag));
     return objectElement;
 }
 
 } // namespace blink
 
 #endif // HTMLObjectElement_h