Refactoring spellcheck-disable-enable.html to use async path for spellcheck

Use asynchronous spellchecking in spellcheck-disable-enable.html.

In addition, use more descriptive variable names and simplify the test.
Automatically fill in the misspellings to the editable fields so
non DumpRenderTree users don't have to do that.

BUG=295722

Review URL: https://codereview.chromium.org/70073005

git-svn-id: svn://svn.chromium.org/blink/trunk@162125 bbb929c8-8fbe-4397-9dbb-9b2b20218538