Revert of Base: add Optional<T>.

docs/optional.md

-# base::Optional
-
-`base::Optional<T>` is a container that might contain an instance of `T`.
-
-[TOC]
-
-## History
-
-[base::Optional<T>](https://code.google.com/p/chromium/codesearch#chromium/src/base/optional.h)
-is an implementation of [std::optional<T>](http://en.cppreference.com/w/cpp/utility/optional),
-initially a C++ experimental feature and now part of the C++17 standard. The
-Chromium's implementation is as close as possible to the specification. The
-differences are listed at the beginning of the header. The most important
-difference is that all the objects and types are part of the `base::` namespace
-instead of `std::`. Also, following Chromium coding style, the class is named
-`Optional` instead of `optional`.
-
-## API description
-
-For a deep API description, please have a look at [std::optional<T>](http://en.cppreference.com/w/cpp/utility/optional)
-or the [Chromium implementation](https://code.google.com/p/chromium/codesearch#chromium/src/base/optional.h).
-
-When initialized without a value, `base::Optional<T>` will be empty. When empty,
-the `operator bool` will return `false` and `value()` should not be called. An
-empty `base::Optional<T>` is equal to `base::nullopt_t`.
-```C++
-base::Optional<int> opt;
-opt == true; // false
-opt.value(); // illegal, will DCHECK
-opt == base::nullopt_t; // true
-```
-
-To avoid calling `value()` when an `base::Optional<T>` is empty, instead of
-doing checks, it is possible to use `value_or()` and pass a default value:
-```C++
-base::Optional<int> opt;
-opt.value_or(42); // will return 42
-```
-
-It is possible to initialize a `base::Optional<T>` from its constructor and
-`operator=` using `T` or another `base::Optional<T>`:
-```C++
-base::Optional<int> opt_1 = 1; // .value() == 1
-base::Optional<int> opt_2 = base::Optional<int>(2); // .value() == 2
-```
-
-All basic operators should be available on `base::Optional<T>`: it is possible
-to compare a `base::Optional<T>` with another or with a `T` or
-`base::nullopt_t`.
-```C++
-base::Optional<int> opt_1;
-base::Optional<int> opt_2 = 2;
-
-opt_1 == opt_2; // false
-opt_1 = 1;
-
-opt_1 <= opt_2; // true
-opt_1 == 1; // true
-opt_1 == base::nullopt_t; // false
-```
-
-`base::Optional<T>` has a helper function `make_optional<T&&>`:
-```C++
-base::Optional<int> opt = make_optional<int>(GetMagicNumber());
-```
-
-Finally, `base::Optional<T>` is integrated with `std::hash`, using
-`std::hash<T>` if it is not empty, a default value otherwise. `.emplace()` and
-`.swap()` can be used as members functions and `std::swap()` will work with two
-`base::Optional<T>` objects.
-
-## How is it implemented?
-
-`base::Optional<T>` is implemented using `base::AlignedMemory`. The object
-doesn't behave like a pointer and doesn't do dynamic memory allocation. In
-other words, it is guaranteed to have an object allocated when it is not empty.
-
-## When to use?
-
-A very common use case is for classes and structures that have an object not
-always available, because it is early initialized or because the underlying data
-structure doesn't require it.
-
-It is common to implement such patterns with dynamically allocated pointers,
-`nullptr` representing the absence of value. Other approaches involve
-`std::pair<T, bool>` where bool represents whether the object is actually
-present.
-
-It can also be used for simple types, for example when a structure wants to
-represent whether the user or the underlying data structure has some value
-unspecified, a `base::Optional<int>` would be easier to understand than a
-special value representing the lack of it. For example, using -1 as the
-undefined value when the expected value can't be negative.
-
-## When not to use?
-
-It is recommended to not use `base::Optional<T>` as a function parameter as it
-will force the callers to use `base::Optional<T>`. Instead, it is recommended to
-keep using `T*` for arguments that can be ommited, with `nullptr` representing
-no value.
-
-Furthermore, depending on `T`, MSVC might fail to compile code using
-`base::Optional<T>` as a parameter because of memory alignment issues.