We need this for isolates testing. To make it work I had to extend the
internal Thread constructor with an option to set the stack size (see
the comment in shell.cc).

BUG=1264

Review URL: http://codereview.chromium.org/6711068

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7289 ce2b1a6d-e550-0410-aec6-3dcde31c8c00