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To support tracing, a tracing service must also be registered with
the mojo broker.

This change does not implement start-up tracing which may be addressed
at a later time.

If (GN arg) enable_external_mojo_tracing=false (default), then a no-op
tracing client implementation will be used.

Bug: internal b/153566474
Test: Tested tracing with an external tracing service.
Change-Id: I56ccecde888778a3f9a54353bb3e6744fb8194c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2200081Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Chris Letnick <cletnick@google.com>
Cr-Commit-Position: refs/heads/master@{#770734}