The sweeper thread should not write the page flags. Added a sweeping complete phase, where the main thread writes the given page flags.

BUG=
R=jarin@chromium.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/163683003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19377 ce2b1a6d-e550-0410-aec6-3dcde31c8c00