Reapply r10158: Distinguish weak references in heap snapshots, group GC roots.

Reapply r10158: Distinguish weak references in heap snapshots, group GC roots.

[Fixed the bug revealed by the Win32 bot]

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*
TBR=vitalyr@chromium.org

Initial CL: http://codereview.chromium.org/8716009

Committed: http://code.google.com/p/v8/source/detail?r=10182