Reduce signal sender thread stack size to 32k.
Commit partial old-space pages to reduce minimum memory use.
This is a rebase of http://codereview.chromium.org/9017009/
Review URL: http://codereview.chromium.org/9179012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10413 ce2b1a6d-e550-0410-aec6-3dcde31c8c00