This function was implemented in a few classes but is a common element
in all form associated elements and the functionality should be there.
With these minimal changes we get to implement 4 idl functions for free.
This change — part of the HTML constraint-validation API (aka
“client-side form validation”) — implements the willValidate IDL/DOM
attribute/property for all form controls that support it.
Note that these aren't full implementations of the bindings. This
mostly implements the low hanging fruit (namely, basic reflections)
There are some attributes that should be USVString instead of
DOMString. However, USVString is a slightly different definition
of DOMString, so it should suffice for now.
