memory-infra: move VM_REGIONS_ONLY from a type to level of detail

VM_REGIONS_ONLY never made sense as a type. It was talking about what
needed to be returned which is captured by level of detail rather than
the type.

With the big refactoring of the whole memory dump arguments incoming,
moving this clears the path for working on more complex issues in the
system.

Bug: 775034
Bug: 777820
Change-Id: If2ada9c4dd3cbd25a1c5b49dec2ec3e68f9c4f95
Reviewed-on: https://chromium-review.googlesource.com/735323
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Commit-Queue: Lalit Maganti <lalitm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511221}

• base/trace_event/memory_dump_request_args.cc[diff]
• base/trace_event/memory_dump_request_args.h[diff]
• base/trace_event/memory_dump_scheduler.cc[diff]
• services/resource_coordinator/memory_instrumentation/coordinator_impl.cc[diff]
• services/resource_coordinator/memory_instrumentation/queued_request.h[diff]
• services/resource_coordinator/memory_instrumentation/queued_request_dispatcher.cc[diff]
• services/resource_coordinator/public/cpp/memory_instrumentation/memory_instrumentation_struct_traits.cc[diff]
• services/resource_coordinator/public/interfaces/memory_instrumentation/memory_instrumentation.mojom[diff]