Summary:
MemLab could run out of memory if the JS heap under analysis has complex code heap objects (i.e., JS interpreted, and compiled code representation and their context graph in heap) which may take a lot of time and memory to serialize:
https://github.com/facebook/memlab/issues/122

This diff fixes this issue by simplifying the code heap object serialization process and output.

Reviewed By: tulga1970

Differential Revision: D59541476

fbshipit-source-id: a4eb00e0f914d4694a1fe562abe0c861ca641ad1