This removes the isolate=>heap=>isolate nonsense and has the additional bonus
that it re-enables printing of code objects in GDB. NOT: To make the latter
work, one has to adapt GDB any macros using FindCodeObject! Keeping things as it
is and outlining Isolate::heap() was not really an option...

Side note: Currently we are lucky that we still have Isolate::Current()
available in GDB, although it is marked as INLINE. :-}

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/19785004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15770 ce2b1a6d-e550-0410-aec6-3dcde31c8c00