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Mojo messages are already covered by toplevel.flow.
Move legacy IPC there as well to centralise tracing for all async
work in a single place.

R=skyostil@chromium.org,rockot@google.com

Change-Id: Iba3d3a25bbb8bb05a8a7e5617312415e15126371
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1985970Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730222}