Updated position: sticky support of Chrome, Opera

Support for sticky positioning, which was disabled by default anyway, was removed in Chrome 38 and Opera 25 (the Opera version is the Chrome version minus 13).
https://groups.google.com/a/chromium.org/forum/#!searchin/blink-checkins/sticky%7Csort:date/blink-checkins/Oedyb7NByWI/WRbvDH3_Yl0J