Add support of Maybe<void> as a template specialization.

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}

include/v8.h

@@ -8423,18 +8423,45 @@ class Maybe {
   friend Maybe<U> Just(const U& u);
 };
 
-
 template <class T>
 inline Maybe<T> Nothing() {
   return Maybe<T>();
 }
 
-
 template <class T>
 inline Maybe<T> Just(const T& t) {
   return Maybe<T>(t);
 }
 
+// A template specialization of Maybe<T> for the case of T = void.
+template <>
+class Maybe<void> {
+ public:
+  V8_INLINE bool IsNothing() const { return !is_valid_; }
+  V8_INLINE bool IsJust() const { return is_valid_; }
+
+  V8_INLINE bool operator==(const Maybe& other) const {
+    return IsJust() == other.IsJust();
+  }
+
+  V8_INLINE bool operator!=(const Maybe& other) const {
+    return !operator==(other);
+  }
+
+ private:
+  struct JustTag {};
+
+  Maybe() : is_valid_(false) {}
+  explicit Maybe(JustTag) : is_valid_(true) {}
+
+  bool is_valid_;
+
+  template <class U>
+  friend Maybe<U> Nothing();
+  friend Maybe<void> JustVoid();
+};
+
+inline Maybe<void> JustVoid() { return Maybe<void>(Maybe<void>::JustTag()); }
 
 /**
  * An external exception handler.