Verify spellcheck attribute asynchronously

Refactoring spelling tests that verify spellcheck attribute to use
asynchronous text checking.

Additionally, add the elements that are being tested to to the tree.
It gives a possibility for non-DumpRenderTree users to see misspellings.

Fix the description message according to the test behavior.

This change is based on WebKit changeset: http://trac.webkit.org/changeset/163432

BUG=295722

Review URL: https://codereview.chromium.org/190663014

git-svn-id: svn://svn.chromium.org/blink/trunk@168998 bbb929c8-8fbe-4397-9dbb-9b2b20218538