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This is a reland of 4df8a2dd

Original change's description:
> Add a Mojo-based transportation layer for Perfetto
>
> This adds a base layer that individual logging protocols (like tracing)
> can use to send their data to Perfetto, using Mojo messaging and
> shared memory.
>
> First CL of several, after this comes the Chrome tracing layer: https://chromium-review.googlesource.com/c/chromium/src/+/981270
>
> A preview of the full integration can be seen here: https://chromium-review.googlesource.com/c/chromium/src/+/852808
>
> The design doc for the integration: https://docs.google.com/document/d/1a8lvN3zYWEyAsBjtuRRsy0fWOWdUdbw6bGnKLBscGdw/edit
>
> R=primiano@chromium.org,skyostil@chromium.org
>
> Change-Id: Ie1db5745ebffdcf1e2d74e2f6b12395f447c8552
> Reviewed-on: https://chromium-review.googlesource.com/980578
> Reviewed-by: oysteine <oysteine@google.com>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Reviewed-by: Primiano Tucci <primiano@chromium.org>
> Commit-Queue: oysteine <oysteine@google.com>
> Cr-Commit-Position: refs/heads/master@{#551019}

Change-Id: Ic47afdb6c2e09924a5f540fa56552ceb6e29e6d7

TBR=dcheng@chromium.org,primiano@chromium.org,oysteine@google.com,skyostil@chromium.org

Change-Id: Ic47afdb6c2e09924a5f540fa56552ceb6e29e6d7
Reviewed-on: https://chromium-review.googlesource.com/1015189Reviewed-by: oysteine <oysteine@chromium.org>
Commit-Queue: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551526}