This is neutral on in-browser SunSpider, but beneficial on other
things, and is likely to lower memory use by collecting earlier.
Review URL: http://codereview.chromium.org/5753003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5968 ce2b1a6d-e550-0410-aec6-3dcde31c8c00