Convert OSExchangeData virtual filenames to use std::optional

Bug: 328710562
Change-Id: I193613f9d52dc15b36db10a551884501099a22e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5352916


Commit-Queue: Avi Drissman <avi@chromium.org>
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1270645}