Add a comment about why HTMLObjectElement inherits FormAssociatedElement

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

Index: third_party/WebKit/Source/core/html/HTMLObjectElement.h
diff --git a/third_party/WebKit/Source/core/html/HTMLObjectElement.h b/third_party/WebKit/Source/core/html/HTMLObjectElement.h
index e1eedf6dbec591dba2ad778aa2781245d8c4758e..6721fc9cac52584aa516ff93dab8ba3d0737e202 100644
--- a/third_party/WebKit/Source/core/html/HTMLObjectElement.h
+++ b/third_party/WebKit/Source/core/html/HTMLObjectElement.h
@@ -31,6 +31,9 @@ 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);