Move the management of the already swept pages to MarkCompactCollector

That way, MCC doesn't need to know about the state the threads are in to
steal free memory from them.

BUG=v8:3104
R=hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/143283002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18706 ce2b1a6d-e550-0410-aec6-3dcde31c8c00