This is the heap representing system memory. The memory type
corresponding to `MTLStorageModeShared` is reported as belonging to this
heap.
I've tried to correct the reported size of the heaps as well while I'm
at it. UMA GPUs and the system heap for NUMA GPUs use the system memory
size as the heap size, and the amount of available memory as the budget.
iOS 13 uses the new `os_proc_available_memory()` API for this purpose.
NUMA GPUs additionally use the memory used by the process as a crude
measure of system heap usage.
