Use base::span in webusb_descriptors.h

This avoids a copy but mostly just makes code calling these methods
cleaner because they don't need to use the much harder to call
std::vector constructor.

Change-Id: I3b91c4ae51960b859b30aacdfd26f6056252c2a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3526182


Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#981397}