Otherwise, retaned memory sizes are not precise. This increases size
of heap snapshot, I will deal with this later. Heap objects and
references previously missing in snapshot are now marked as 'hidden'.
That means, they not shown to user, but participate in sizes
calculation.

Other small changes:
 - added 'shortcut' graph edges: e.g. to pin global objects on top
   level;
 - meta-information in JSON snapshot is no more double encoded.

Review URL: http://codereview.chromium.org/5139002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5849 ce2b1a6d-e550-0410-aec6-3dcde31c8c00