After this change, you can just call DumpBacktrace in any function. Previously
the only function printing stack traces was V8_Fatal, but that also terminated
the program, so not very useful for debugging.

Note that DumpBacktrace is roughly equivalent to base::debug::StackTrace
functionality in Chromium, except less fancy, but it's enough for us.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/200343003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19993 ce2b1a6d-e550-0410-aec6-3dcde31c8c00