Introduce rtc::Maybe<T>, which either contains a T or not.

It's a simple std::experimental::optional-wannabe. For simplicity and
portability, it still secretly contains a (default-constructed) T when
it's supposedly empty. This restriction is fine for simple types.

One important application is for the return type of functions. For
example, a function which either returns a size_t or fails can return
rtc::Maybe<size_t>.

BUG=webrtc:5028
R=andrew@webrtc.org, mgraczyk@chromium.org

Review URL: https://codereview.webrtc.org/1413763003 .

Cr-Commit-Position: refs/heads/master@{#10353}