Bug 1380415 part 2. Remove XPCOM constraint validation APIs. r=mccr8

dom/html/HTMLButtonElement.h

@@ -163,13 +163,9 @@ public:
     SetHTMLAttr(nsGkAtoms::value, aValue, aRv);
   }
 
-  // nsIConstraintValidation::WillValidate is fine.
-  // nsIConstraintValidation::Validity() is fine.
-  // nsIConstraintValidation::GetValidationMessage() is fine.
-  // nsIConstraintValidation::CheckValidity() is fine.
-  using nsIConstraintValidation::CheckValidity;
-  using nsIConstraintValidation::ReportValidity;
-  // nsIConstraintValidation::SetCustomValidity() is fine.
+  // Override SetCustomValidity so we update our state properly when it's called
+  // via bindings.
+  void SetCustomValidity(const nsAString& aError);
 
 protected:
   virtual ~HTMLButtonElement();