Background context: go/memory-infra

Background:
- Up until recently PartitionAlloc was living in blink (third_party/WebKit)
- palmer@ did some heroic work and moved it to //base. Right now blink is still the only user but the situation might change at some poin in the future.
- We have a memory-infra probe that reports the memory used by partition alloc in WebKit/Source/platform/PartitionAllocMemoryDumpProvider.cpp

Problem:
1) That PartitionAllocMemoryDumpProvider today knows only about the blink heaps (which also happen to be the only PA heaps)
2) It is triggered only in the renderer . We might use PA in the browser side at some point in the future.
In essence, if in future we get more clients of partition alloc, their memory usage won't be reported anywhere.

Possible solution:
I think we need to split the PartitionAllocMemoryDumpProvider into two pieces: a more generic infrastructure in base, sitting next to PA, and a MemoryDumpProvider which knows and dumps the blink heaps in webkit.

Not high prio right now, but at some point we'll need to do this