Fix for Clusterfuzz issue 343928.

The problem was that the debugger didn't expect that a JSFunction could
have a GlobalContext, which it can with harmony scoping.

BUG=343928
R=yangguo@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/183103003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19576 ce2b1a6d-e550-0410-aec6-3dcde31c8c00