This allows base::nullopt to be used instead of base::Optional<Foo>()
and implicit conversion to Optional<T> from T.

Also added NOLINT comments to the implicit constructors.

Change-Id: I4c688045685e2a50e0c0d38a959570f77454ec61
Reviewed-on: https://chromium-review.googlesource.com/893268Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51003}