Make position: sticky a runtime enabled CSS feature

--enable-experimental-web-platform-features enables it on
Chromium side so this shouldn't change when it is enabled
but helps remove some unneeded custom code.

Review URL: https://codereview.chromium.org/25683013

git-svn-id: svn://svn.chromium.org/blink/trunk@158973 bbb929c8-8fbe-4397-9dbb-9b2b20218538