Implement heap snapshots serialization into JSON. API is designed

Implement heap snapshots serialization into JSON. API is designed
to avoid storing serialized snapshot on VM, instead it is emitted
using output stream interface.

The size of JSON emitted is roughly equal to used heap size
(when stored as an ASCII string).

Now a whole heap snapshot can be serialized and transmitted outside
VM. This makes possible:
  - implementing non-async UI for heap snapshots inspection;
  - storing heap snapshots for further inspection;
  - remote profiling (we can even implement a snapshotting mode
    where a snapshot isn't even stored in VM, only transmitted --
    good for mobile devices);
  - creating tools for outside heap snapshots processing, e.g.
    converting to HPROF.

Review URL: http://codereview.chromium.org/3311028

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5450 ce2b1a6d-e550-0410-aec6-3dcde31c8c00