The HTMLElementFactory used to take the form element pointer for form
association. That operation happens as a discrete step now for
customized built-in elements. Now that HTMLElementFactory doesn't do
form association any more, HTML and non-HTML element construction can
be merged together the way they are in the specs.

BUG=648828

Review-Url: https://codereview.chromium.org/2842933002
Cr-Commit-Position: refs/heads/master@{#467272}