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This change wraps strong references to libusb_device objects in a
ScopedLibusbDevice class (specializing ScopedGeneric) in order to better
document when these references are expected to be dropped.

Bug: 819356
Change-Id: I9671d340f0d5353637405ddd72846456e23a5f1f
Reviewed-on: https://chromium-review.googlesource.com/972623Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545620}