Add a cursor iteration test for prevunique that crashes with current code, but is fixed by https://codereview.chromium.org/368873005/.

BUG=372103

Review URL: https://codereview.chromium.org/363143002

git-svn-id: svn://svn.chromium.org/blink/trunk@177619 bbb929c8-8fbe-4397-9dbb-9b2b20218538