Use PropertySet in place of HashSet<CSSPropertyID>.

No functional change.

BUG=240653
R=dstockwell

Review URL: https://chromiumcodereview.appspot.com/23766041

git-svn-id: svn://svn.chromium.org/blink/trunk@158345 bbb929c8-8fbe-4397-9dbb-9b2b20218538