The documentation was referring to the struct base::nullopt_t
where it should have been using the constexpr base:nullopt instead,
leading to confusing compiler errors when following the examples.

Also add an example showing how to pass an empty optional argument,
since this wasn't immediately obvious to me.

R=mlamouri

  constexpr nullopt_t nullopt(0);

https: //cs.chromium.org/chromium/src/base/optional.h defines:
Change-Id: I4fc246ebd5282794e44d88587336f2190250350d
Reviewed-on: https://chromium-review.googlesource.com/662982


Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501682}