Bug 1358448 - Add radio back to a radio group after moving out of a form. r=smaug

The container of a radio group is the form, if it belongs to a form, or the
document object otherwise.
When moving a radio out of a form, we should add it back to a radio group.
Similary, before moving the radio to a form, we should remove it from the
original radio group.

MozReview-Commit-ID: 22WsEhz2SXH

dom/html/HTMLInputElement.h

@@ -984,6 +984,10 @@ protected:
   virtual nsresult AfterSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
                                 const nsAttrValue* aValue, bool aNotify) override;
 
+  virtual void BeforeSetForm(bool aBindToTree) override;
+
+  virtual void AfterClearForm(bool aUnbindOrDelete) override;
+
   /**
    * Dispatch a select event. Returns true if the event was not cancelled.
    */