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This is an initial implementation that currently requires that all
tests run on a thread allowed to do IO operations. A follow-up CL will
focus on moving all the blocking operations to a separate thread.

BUG=773382

Change-Id: Iea5ee2ede75f4fff168f6546b515018f0fd2a0ca
Reviewed-on: https://chromium-review.googlesource.com/1005444
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557611}