Clean up a few more remnants of absl::optional

There were some remaining includes of the Abseil header. Also a few
files under //third_party were missed. (These are bits of //third_party
which depend on //base, so we should be able to apply Chromium's
expectations. E.g. files we override in WebRTC to inject a //base
dependency, or workarounds for Chromium's test infrastructure
limitations.)

Bug: 40242125
Change-Id: I6c442fe6ae28da08b094044e80973a581e60cd71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5691505
Reviewed-by: danakj <danakj@chromium.org>
Owners-Override: danakj <danakj@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1326396}