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We will eventually support detecting printers using mDNS and autodetect
USB devices.  This will take a little while to implement.  Providing a
fake in the meantime since the system is behind a flag.  The fake may be
useful for some tests in the future.

BUG=631352

Review-Url: https://codereview.chromium.org/2344393002
Cr-Commit-Position: refs/heads/master@{#422641}