Form-associated custom element: Fix a form state restore issue

This CL fixes a bug that restoring state of a <form> fails if
 - The form contains a form-associated custom element as the first or
   the second associated control.
 - When the form is parsed, the custom element definition for the
   form-associated custom element is not available yet.

We generate an identifier string for a <form>, and it contains names of
associated controls. So it depended on a upgrade timing of a
form-associated custom element. This CL skip names of form-associated
custom elements on form identifier generation.

Bug: 905922
Change-Id: I6f52967880b62425c118c31e364213d74ed4bb70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1583171
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653965}
3 files changed