Two small refinements to matched properties cache.

Two small refinements to matched properties cache.
<https://webkit.org/b/125992>

- Avoid computing the matched properties hash if we're banned from
  using the cache anyway.

- When adding a new entry to the cache, use move semantics to avoid
  creating a transient copy of all the data.

Reviewed by Antti Koivisto.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@160829 268f45cc-cd09-0410-ab3c-d52691b4dbfc