Use acquire/release stores and loads when reading and writing the parallel sweeping state of a page.

BUG=
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/214543008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20301 ce2b1a6d-e550-0410-aec6-3dcde31c8c00