[mojo] Add multiprocess test to DataPipe.
[mojo] Add multiprocess test to DataPipe.

This is in preparation for a CL modifying DataPipe to use shared buffers to transfer data.

BUG=556258

Review URL: https://codereview.chromium.org/1537603002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#368736}

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