Form-associated custom elements: Support the |anchor| argument of setValidity()

This CL follows the latest specification PR;
  https://github.com/whatwg/html/pull/4383

Add the |anchor| argument to setValidity().  ElementInternals has a data
member to store it.  ListedElement shows a validation bubble on
ValidationAnchor() result, which returns the data member if it's not
null, returns the target element otherwise.

Internals::isValidationMessageVisible() is updated so that it asks
ValidationMessageClient directly because we can show validation bubble
on any elements.

Bug: 905922
Change-Id: I20e1bdfeed6ab2635fd3f171e5160feeba170681
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614644
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661295}
11 files changed