Empty but unswept pages could cause the unswept_free_bytes counter to
to be off in case heap gets shrunk and page gets released before it was
swept properly.

R=vegorov@chromium.org
BUG=v8:1893

Review URL: https://chromiumcodereview.appspot.com/9241010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10421 ce2b1a6d-e550-0410-aec6-3dcde31c8c00