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Add a tracing agent and matching service so that tracing on Cast devices
can collect kernel traces. The implementation is based on the CrOS
tracing agent and debugd service.

This is very barebones at the moment (service only depends on //base) to
keep the size down. We don't have dbus, although we might be able to use
mojo to manage the socket.

BUG=786091

Change-Id: I76157157506628004d812db3f452a486b2962845
Reviewed-on: https://chromium-review.googlesource.com/775557Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521036}