New interface and function for form association

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

Index: third_party/WebKit/Source/core/html/HTMLElement.h
diff --git a/third_party/WebKit/Source/core/html/HTMLElement.h b/third_party/WebKit/Source/core/html/HTMLElement.h
index 9ad60ed559ed3b682f782e35b2d26e7db18a71bb..342d8df69316078fc1f995ab0c6f7366b8f02af1 100644
--- a/third_party/WebKit/Source/core/html/HTMLElement.h
+++ b/third_party/WebKit/Source/core/html/HTMLElement.h
@@ -29,9 +29,10 @@
 namespace blink {
 
 class DocumentFragment;
+class ExceptionState;
+class FormAssociated;
 class HTMLFormElement;
 class HTMLMenuElement;
-class ExceptionState;
 class KeyboardEvent;
 
 enum TranslateAttributeMode {
@@ -119,6 +120,8 @@ class CORE_EXPORT HTMLElement : public Element {
 
   Element* unclosedOffsetParent();
 
+  virtual FormAssociated* toFormAssociated() { return nullptr; };

[tkent, 2016/12/05 01:53:50]

We avoid to add such conversion functions to Element because there are too many
such conversions.
Please
 - Add HTMLElement::isFormAssociated, and HTMLImageElement,
HTMLFormControlElement, and HTMLObjectElement should implement it so that it
returns |true|.
 - Add DEFINE_TYPE_CAST(FormAssociated, HTMLElement, element,
element->isFormAssociated(), element.isFormAssociated()); to FormAssociated.h
 - In HTMLConstructionSite.cpp,
  if (isFormAssociated(element))
    toFormAssociated(element)->setAssociatedForm(form);

+
  protected:
   HTMLElement(const QualifiedName& tagName, Document&, ConstructionType);