Add layout test which checks sticky postioning when overflow is scrolled by fragment.

Related to this w3c mailing list thread:
http://lists.w3.org/Archives/Public/www-style/2014Mar/0013.html

BUG=231754

Review URL: https://codereview.chromium.org/186943003

git-svn-id: svn://svn.chromium.org/blink/trunk@168528 bbb929c8-8fbe-4397-9dbb-9b2b20218538