Oilpan: Prepare to move FormAssociatedElement to Oilpan heap, part 1.

FormAssociatedElement should be GarbageCollectedMixin, and this CL replaces
existing RefPtr<FormAssociatedElement> with Oilpan transition types.

This CL doesn't handle existing FormAssociatedElement raw pointers. The next CL
will handle them.

This CL moves ValidityState to Oilpan heap because ValidityState depends on
FormAssociatedElement::ref() and deref().

BUG=357163

Review URL: https://codereview.chromium.org/271533009

git-svn-id: svn://svn.chromium.org/blink/trunk@173634 bbb929c8-8fbe-4397-9dbb-9b2b20218538