Add a mode to js_merger that prints out how many things are merged, rather than the address therein
Add new mode that only prints out the address of the rooting object for each path.

The idea is that the browser could invoke this, then start refcount logging these objects.
In roots-only mode, don't print out when we don't find roots for something
in the helper find_roots script, look at the base of the file name to decide whether it is a GC or CC log