headless: Replace headless::Maybe with base::Optional

headless/public/util/maybe.h

-// Copyright 2016 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef HEADLESS_PUBLIC_UTIL_MAYBE_H_
-#define HEADLESS_PUBLIC_UTIL_MAYBE_H_
-
-#include <algorithm>
-
-#include "base/logging.h"
-#include "base/macros.h"
-
-namespace headless {
-
-// A simple Maybe which may or may not have a value. Based on v8::Maybe.
-// TODO(skyostil): Replace this with base::Optional once it is available.
-template <typename T>
-class Maybe {
- public:
-  Maybe() : has_value_(false) {}
-
-  bool IsNothing() const { return !has_value_; }
-  bool IsJust() const { return has_value_; }
-
-  // Will crash if the Maybe<> is nothing.
-  T& FromJust() {
-    DCHECK(IsJust());
-    return value_;
-  }
-  const T& FromJust() const {
-    DCHECK(IsJust());
-    return value_;
-  }
-
-  T FromMaybe(const T& default_value) const {
-    return has_value_ ? value_ : default_value;
-  }
-
-  bool operator==(const Maybe& other) const {
-    return (IsJust() == other.IsJust()) &&
-           (!IsJust() || FromJust() == other.FromJust());
-  }
-
-  bool operator!=(const Maybe& other) const { return !operator==(other); }
-
-  Maybe& operator=(Maybe&& other) {
-    has_value_ = other.has_value_;
-    value_ = std::move(other.value_);
-    return *this;
-  }
-
-  Maybe& operator=(const Maybe& other) {
-    has_value_ = other.has_value_;
-    value_ = other.value_;
-    return *this;
-  }
-
-  Maybe(const Maybe& other) = default;
-  Maybe(Maybe&& other) = default;
-
- private:
-  template <class U>
-  friend Maybe<U> Nothing();
-  template <class U>
-  friend Maybe<U> Just(const U& u);
-  template <class U>
-  friend Maybe<typename std::remove_reference<U>::type> Just(U&& u);
-
-  explicit Maybe(const T& t) : has_value_(true), value_(t) {}
-  explicit Maybe(T&& t) : has_value_(true), value_(std::move(t)) {}
-
-  bool has_value_;
-  T value_;
-};
-
-template <class T>
-Maybe<T> Nothing() {
-  return Maybe<T>();
-}
-
-template <class T>
-Maybe<T> Just(const T& t) {
-  return Maybe<T>(t);
-}
-
-template <class T>
-Maybe<typename std::remove_reference<T>::type> Just(T&& t) {
-  return Maybe<typename std::remove_reference<T>::type>(std::move(t));
-}
-
-}  // namespace headless
-
-#endif  // HEADLESS_PUBLIC_UTIL_MAYBE_H_