Blink wants to use Maybe<T> as a return type of (author) callback
functions, where T can be type void.  So, this patch adds support
of Maybe<void>.

Bug: chromium:778580, chromium:779036
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Id654bafc5ceac8ef6f755902418f250c353a8837
Reviewed-on: https://chromium-review.googlesource.com/771730
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49416}