Upgrade TargetedStyleRecalc to status=stable.

The feature has been at status=experimental for over a month, and all known bugs have been fixed.

BUG=335247

Review URL: https://codereview.chromium.org/208153007

git-svn-id: svn://svn.chromium.org/blink/trunk@169765 bbb929c8-8fbe-4397-9dbb-9b2b20218538