Verify misspellings after lines merge/split asynchronously

Adapt the following spelling tests to use asynchronous spellchecking:
    - editing/spelling/spelling-backspace-between-lines.html
    - editing/spelling/spelling-linebreak.html

Group test cases into separated methods. Use shouldBeEqualToString to
compare strings. Add newly created div elements instead of overriding
the previous one in order that non DumpRenderTree users can see what
has been tested.

It's based on WebKit changeset: http://trac.webkit.org/changeset/163615

BUG=295722

Review URL: https://codereview.chromium.org/197403002

git-svn-id: svn://svn.chromium.org/blink/trunk@169222 bbb929c8-8fbe-4397-9dbb-9b2b20218538