src: add --track-heap-objects

- This makes v8 add .trace_function_info to the serialized form of
  snapshots from v8::HeapSnapshot::Serialize
- .trace_funciton_info combined with .trace_node in snapshots tells the
  JS location that allocated a specific object

PR-URL: https://github.com/nodejs/io.js/pull/2135


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>