Use an std::set for keeping track of CSS order values.

Previously we would store the values in a Vector and sort it.
This will save memory by not needing to keep the Vector and it
is simpler.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/24632003

git-svn-id: svn://svn.chromium.org/blink/trunk@158388 bbb929c8-8fbe-4397-9dbb-9b2b20218538