Reland "Add a trace event when a task scheduler worker is active."

This is a reland of d641a6adecc1dddf67823f2b424b6ebac79bce02.

It was reverted per causing data races in MemoryDumpManagerTests
as they re-initialized the MemoryDumpManager instance after scheduler
workers had been brought up (and the workers using tracing prior to
receiving a task -- which previously acted as a sync point -- is
now highlighting this initialization race).

Original change's description:
> Add a trace event when a task scheduler worker is active.
>
> Example trace with this enabled: https://drive.google.com/file/d/11si1qvQUhRi1jMPNTzgJpqp3D-yFKOf7/view?usp=sharing
>
> Also fix a racy setup order in MemoryDumpManagerTest exercised by this change.
>
> R=fdoray@chromium.org
>
> Bug: 811035
> Change-Id: I328dca43f7b5bfd42bcf1a69b1e191fe41523ade
> Reviewed-on: https://chromium-review.googlesource.com/913251
> Reviewed-by: François Doray <fdoray@chromium.org>
> Commit-Queue: Gabriel Charette <gab@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#536177}

Bug: 811035, 811482
Change-Id: If3a7b1dbfa8d50cec97b7b98e88c68f26f9813ec

TBR=fdoray@chromium.org

Change-Id: If3a7b1dbfa8d50cec97b7b98e88c68f26f9813ec
Reviewed-on: https://chromium-review.googlesource.com/915941
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Commit-Queue: Primiano Tucci <primiano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536322}

CrOS-Libchrome-Original-Commit: 8c1fb16b6c55d7fbce3f2c90b56b70e6966b2290
2 files changed
tree: cdc6f28bedb87cc062b00334a24fd7a6bbac1b59
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/