create a new persistent handle to hold the context to save when
entering another context, now we use a stack of direct pointers that
the gc knows about.

Review URL: http://codereview.chromium.org/199021

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2820 ce2b1a6d-e550-0410-aec6-3dcde31c8c00