Another step on the way to context snapshots. We can now refer to

Another step on the way to context snapshots.  We can now refer to
objects in the startup heap from a partial snapshot.  This happens
through the partial snapshot cache.  A startup snapshot and a
partial snapshot are created together so that the startup snapshot
contains the partial snapshot cache entries needed.
Review URL: http://codereview.chromium.org/548149

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3713 ce2b1a6d-e550-0410-aec6-3dcde31c8c00