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* TraceWriters will no longer be destroyed when their TLS slot is cleaned
  up on thread shutdown as this can trigger a CommitData() call from
  Perfetto and cause asserts when Mojo does a TaskRunnerHandle::Get()
  for a PostTask; instead we leave the deletion to the taskrunner
  Perfetto is running on (if it's getting shut down, the task should
  get dropped).
* Run ProducerClients on their own sequence.

Bug: 844379
Change-Id: Ie45b36a4e463674122555dbec2105953a882f37c
Reviewed-on: https://chromium-review.googlesource.com/1069528Reviewed-by: Primiano Tucci <primiano@chromium.org>
Commit-Queue: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561356}