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This change modifies ScopedLibusbDeviceRef and adds a new class
ScopedLibusbDeviceHandle. This now explicitly own a reference to the
UsbContext object (which reference counts a libusb_context) in addition
to the libusb_device or libusb_device_handle they are wrapping.

This resolves potential use-after-frees possible when posting tasks
with a ScopedLibusbDeviceRef since the UsbService the task is being
posted to could be destroyed before the task is executed. The
libusb_device would then be released after its libusb_context has been
destroyed.

This is based on https://crrev.com/c/1131949 after I realized there
where additional issues that needed to be addressed.

Bug: 838947
Change-Id: Idee02828bf615bd477033e585fffe03cf4d20595
Reviewed-on: https://chromium-review.googlesource.com/1145910Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577329}