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
Move the temporary private freelist closer to where it's used

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

Review URL: https://codereview.chromium.org/143193002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18707 ce2b1a6d-e550-0410-aec6-3dcde31c8c00