This changes the heuristic that starts incremental marking to be based
on a more accurate heap size estimation. Pages being swept lazily can be
accounted using the live bytes counter.

R=mstarzinger@chromium.org
BUG=v8:1682

Review URL: https://chromiumcodereview.appspot.com/9674001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11004 ce2b1a6d-e550-0410-aec6-3dcde31c8c00