Distinguish weak references in heap snapshots, group GC roots.

Distinguish weak references in heap snapshots, group GC roots.

Several changes to better organize snapshot data:

1. Provide information about weak references.
2. Group (GC roots) children.
3. Prettify debug snapshot printing.

BUG=v8:1832
TEST=cctest/test-heap-profiler/*Weak*

Review URL: http://codereview.chromium.org/8716009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10158 ce2b1a6d-e550-0410-aec6-3dcde31c8c00