Adapt hidpi spelling tests to use async path for spellcheck

Apply changes from https://codereview.chromium.org/49613006/ to
inline-spelling-markers-hidpi.html and
editing/spelling/inline-spelling-markers-hidpi-composited.html.

BUG=295722

Review URL: https://codereview.chromium.org/57823002

git-svn-id: svn://svn.chromium.org/blink/trunk@161439 bbb929c8-8fbe-4397-9dbb-9b2b20218538