Refactoring spellcheck-attribute.html to use async path for spellcheck

Deprecate synchronous spellchecking in spellcheck-attribute.html
in a favor of its asynchronous option.

BUG=295722

Review URL: https://codereview.chromium.org/58953005

git-svn-id: svn://svn.chromium.org/blink/trunk@161898 bbb929c8-8fbe-4397-9dbb-9b2b20218538