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[usb] Remove out parameters in UsbServiceWin helper functions

As pointed out in [1] the style guide recommends against out parameters.
This change rewrites most of the helper functions in UsbServiceWin to
return their value directly, using an empty string or nullopt as a
sentinel for failure.

[1]: https://chromium-review.googlesource.com/c/chromium/src/+/2132532

Bug: 637404
Change-Id: I54b4b807fae95f9ff93e9a02546e2748cf00b9f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2135129
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756347}