Allowing the total number of objects to vary in GCPerfSim

In order to fix the problem where GCPerfSim is not allocating as many bytes as specified by the arguments, I made a change to the processing of `ReferenceItemWithSize`. In particular, the original code intentionally made multiple allocations in a single `JustMakeObject` call in order to produce some variability in the total number of objects (and therefore the heap size) when the program runs while maintaining the heap size to be more or less close to the specified heap size.

Allowing the total number of objects to vary while also achieving the required allocation ratio is potentially feasible. This issue is filed to track the investigation there.