Add a trap_on_abort flag

By setting this flag assertions behind --debug-code will trigger a
breakpoint instead of a call into Abort. This eases debugging, as the
call site is less cluttered and the backtrace starts where it should.

BUG=
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/23352004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16319 ce2b1a6d-e550-0410-aec6-3dcde31c8c00