This enables custom stack trace formatting for stack overflow.
A consequence is that stack trace formatting is now easily observable,
but we already established that the default stack trace formatting can
be observed anyways. It is only triggered by the .stack getter, and
it has to be explicitly called, (e.g. not implicitly after GC).

R=mstarzinger@chromium.org
BUG=v8:2559

Review URL: https://codereview.chromium.org/20692002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15902 ce2b1a6d-e550-0410-aec6-3dcde31c8c00