Protect os.chdir, os.setenv, os.system against string conversion
failures.  Add comment about the issue to include/v8.h.
Review URL: http://codereview.chromium.org/57005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1644 ce2b1a6d-e550-0410-aec6-3dcde31c8c00