Refactoring markers.html to use async path for spellcheck

Rewrite spelling/markers.html to use asynchronous spellchecking.
Additionally, remove unnecessary code.

BUG=295722

Review URL: https://codereview.chromium.org/59143005

git-svn-id: svn://svn.chromium.org/blink/trunk@161435 bbb929c8-8fbe-4397-9dbb-9b2b20218538