[tracing] Expose level of detail of memory dumps in the trace JSON
[tracing] Expose level of detail of memory dumps in the trace JSON

This small CL exposes a "level_of_detail" field in the trace JSON,
as a sibling of "allocators". e.g.,

"dumps":{
 "allocators":{...},
 "allocators_graph":[...],
 "level_of_detail":"light",
 "process_totals":{...}
}

BUG=524345

Review URL: https://codereview.chromium.org/1344543002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#348861}

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