[tracing] API to express memory suballocation and cross-process sharing

[tracing] API to express memory suballocation and cross-process sharing

- Add a {Get,Create}SharedGlobalAllocatorDump to create shared nodes
  that can be referenced by several processes (concrete use case:
  reporting different details about the same shared memory segment
  from different processes)
- Add a utility AddSuballocation method to express suballocation.
  Under the hoods it creates an anonymous node on the target allocator
  and establishes an ownership relationship with that.

For more details and use cases see the design doc attached to the bug.

BUG=492102

Review URL: https://codereview.chromium.org/1156023007

Cr-Commit-Position: refs/heads/master@{#332581}