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This CL supports setting up mojo connection between lacros and ash in
testing and debugging environment, and it is done via a unix domain
socket.

Unix domain socket is only used in testing, not production because it
allows a random process to connect to ash-chrome as long as it knows
the socket path.

Currently, only one lacros-chrome is allowed to connect to ash-chrome,
so in the testing environment, test launcher will run tests serially,
and will change to run in parallel once ash-chrome supports multiple
clients.

Bug: 1120582
Change-Id: I9736b03218f8fe937a7d642a35d6ee49b7e081f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2384543
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805537}