before performing debugger property lookup make sure the current context is set to the context active before the debugger was entered.

Make the use of the LookupResult GC safe in debugger property lookup.
Review URL: http://codereview.chromium.org/115855

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2071 ce2b1a6d-e550-0410-aec6-3dcde31c8c00