Form-associated custom elements: Update restoreValueCallback

- Stop to make it mandatory
  It didn't make much sense because web developers can provide empty callbacks.

- Add 'mode' argument
  It is 'restore' if the callback called for state restore by UA.
  It will be 'autocomplete' if UA's autofilling feature calls the callback.

Bug: 905922
Bug: https://github.com/w3c/webcomponents/issues/758
Change-Id: I747eb3f1cba97bd9d0b62ed18fd0504fae7250f9
Reviewed-on: https://chromium-review.googlesource.com/c/1438794
Commit-Queue: Kent Tamura <tkent@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627355}
21 files changed