IIUC +lizeb had a plan to move all the known categories to CategoryRegistry (i.e. this [1]).
That would:
1) simplify our job here (just list the CategoryRegistery)
2) Allow us to get rid of that *horrifying* hack where we start and stop a short trace just to discover the categories.

[1] https://cs.chromium.org/chromium/src/base/trace_event/category_registry.cc?sq=package:chromium&dr=CSs&l=29

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ec68fcf2d4026c66c773c1631e8847f065e2eb77

commit ec68fcf2d4026c66c773c1631e8847f065e2eb77
Author: Oystein Eftevaag <oysteine@google.com>
Date: Fri Jul 20 00:35:40 2018

Perfetto: Support enumeration of tracing categories

This re-enables the tracing::AgentRegistry and associated Mojo
connections even when Perfetto is used, which is then used
by the PerfettoTracingController to provide the list of categories to
about://tracing.

Also some cleanup: ServiceFactoryRef is redundant for the tracing
service (it's always running anyway).

Bug:  839065 
Change-Id: Ie094191d16caf10a0400499135b3396ab8595bb2
Reviewed-on: https://chromium-review.googlesource.com/1142724
Commit-Queue: oysteine <oysteine@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576722}
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/agent_registry.cc
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/agent_registry.h
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/agent_registry_unittest.cc
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/coordinator.cc
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/coordinator.h
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/coordinator_unittest.cc
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/perfetto/perfetto_service.cc
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/perfetto/perfetto_service.h
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/perfetto/perfetto_tracing_coordinator.cc
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/perfetto/perfetto_tracing_coordinator.h
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/public/cpp/trace_event_agent.cc
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/public/cpp/trace_event_agent.h
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/public/cpp/trace_event_agent_unittest.cc
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/tracing_service.cc
[modify] https://crrev.com/ec68fcf2d4026c66c773c1631e8847f065e2eb77/services/tracing/tracing_service.h

This bug has an owner, thus, it's been triaged. Changing status to "assigned".