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Adds a new DataSource for the ARC++ system tracing agent.
Each ArcTracingBridge registers with the data source and keeps track of
its state, so that the data source can coordinate tracing between all
the bridges.

TBR=oysteine@google.com

Test: Manually capture a trace with and without perfetto backend.
Bug: 900603
Change-Id: Ieb29bcb65de9f4802965a30844e9f76af55db1f3
Reviewed-on: https://chromium-review.googlesource.com/c/1360736Reviewed-by: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: oysteine <oysteine@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615483}