[tracing] Simplify the MemoryAllocatorDump API

Up until now the API surface of the MemoryAllocatorDump has been
strongly typed, enforcing three basic counters (physical_size,
allocated_objects_count and allocated_objects_size) + arbitrary
extra attributes.
This CL simplifies the MAD interface as follows:
- Remove the two argument (allocator_name, heap_name) in favor of
  a simpler and single argument absolute_name (e.g.,
  CreateAllocatorDump("v8/isolate1/heap2") instead of
  CreateAllocatorDump("v8", "isolate1/heap2"))
- Remove the storngly typed properties in favor of a generic
  Add(attribute_name, type, units, value) method.
- Add helper methods MAD.AddScalar() and AddString().
- Rewrite the existing dumpers to use the new simpler API.
- Fix the tests.

BUG=466141

Review URL: https://codereview.chromium.org/1105143003

Cr-Commit-Position: refs/heads/master@{#327310}

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