Switch AudioTimestampHelper to use std::optional.

There's no reason for this code to have a dependency on the special
kNoTimestamp value we use for playback. It's an artifact of days
before std::optional existed.

R=tmathmeyer

Fixed: 348474005
Change-Id: I569fb4e421c03e96e33167fb341769595502f6ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5648904
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Ted (Chromium) Meyer <tmathmeyer@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1318844}