Add first unit-test for TextIterator.

A new unit test is added in order to help us understand TextIterator's behavior
better. There is only one simple test for now.

BUG=none
R=yosin@chromium.org,morrita@chromium.org

Review URL: https://codereview.chromium.org/60983007

git-svn-id: svn://svn.chromium.org/blink/trunk@161705 bbb929c8-8fbe-4397-9dbb-9b2b20218538