Make IsolationInfo and NetworkIsolationKey take nonces as optionals.

They were being taken as const pointers. This was due to overly strict
adherence to a line from the Google C++ style guide:

    Generally, use std::optional to represent optional by-value inputs,
    and use a const pointer when the non-optional form would have used
    a reference.

While this may make sense in the general case of a function that takes
a value and uses it directly, without more context, it doesn't make
sense in the case of a constructor that stores the input as an
absl::optional, particularly when every since production caller of that
constructor already has the value in an optional. Having all of those
callers have to convert an optional to a pointer, only to have the
constructor covert it back into an optional adds a lot of unnecessary
complexity, and doesn't accomplish anything useful.

BUG: None
Change-Id: I1a51cfab8df385df6fff8ae7dc064e2438740315
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4326990
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Ayu Ishii <ayui@chromium.org>
Reviewed-by: Brianna Goldstein <brgoldstein@google.com>
Cr-Commit-Position: refs/heads/main@{#1118897}