form-associated custom elements: Change mandatory 'value' setter to 'restoreValueCallback' callback.

It was changed due to
https://github.com/w3c/webcomponents/issues/187#issuecomment-450105076

Bug: 905922
Bug: https://github.com/w3c/webcomponents/issues/187
Change-Id: I88952ca5f0aa7d3f48cb2161e5e1d2257e66dfad
Reviewed-on: https://chromium-review.googlesource.com/c/1405951
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622784}
13 files changed