When dumping the stack, try to print contents as ASCII

This makes it easier to find strings that are on the stack for debugging

BUG=none
LOG=n
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/132503005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23045 ce2b1a6d-e550-0410-aec6-3dcde31c8c00