[WTF] Add Markable<T, Traits>

[WTF] Add Markable<T, Traits>
https://bugs.webkit.org/show_bug.cgi?id=189231

Reviewed by Sam Weinig.

Source/WebCore:

Use Markable<Seconds> and Markable<WallTime> in ResourceResponseBase.
Since these fields are parsed results from http header fields, Seconds::nan() and WallTime::nan()
can be used as an empty value for these fields. Thus we can use Markable because it uses
these nan values as an empty values (they are configured by Seconds::MarkableTraits and WallTime::MarkableTraits).
This reduces the size of ResourceResponseBase from 448 to 416.

* platform/network/ResourceResponseBase.h:

Source/WTF:

We can represent a value with nullopt by using std::optional<T>. However, std::optional<T> has storage efficiency
problem. It always has a bool indicating that the value is nullopt or not. If we have a following class,

    class A {
        std::optional<WallTime> m_timeA;
        std::optional<WallTime> m_timeB;
        std::optional<WallTime> m_timeC;
    };

This class has significant amount of padding between m_timeA / m_timeB, m_timeB / m_timeC due to the above bool.

If we know that WallTime has a value that represents invalid, we can use it instead and save the storage.
This is very similar problem to our HashTable implementation. In our HashTable implementation, we need Deleted
and Empty value, which can represent Deleted and Empty values without sacrificing storage efficiency.

We should have similar mechanism here. In this patch, we have WTF::Markable<T, Traits>. Traits offers
`Traits::isEmptyValue(value)` and `Traits::emptyValue()`. Then, we use this empty value instead of having bool
flag. This way, we can make `sizeof(WTF::Markable<T>) == sizeof(T)`.

This idea is inspired from https://github.com/akrzemi1/markable. But we would like to have WTF::Markable<T>
here instead of importing it since we would like to have (1) integrated interfaces with std::optional<T> and (2)
aligned function names to HashTraits' `isEmptyValue` and `emptyValue`.

* WTF.xcodeproj/project.pbxproj:
* wtf/CMakeLists.txt:
* wtf/Markable.h: Added.
(WTF::std::underlying_type<EnumType>::type>::max): EnumMarkableTraits can be used as an MarkableTraits for enum
values. We can specify a constant value as an empty value.
(WTF::IntegralMarkableTraits::isEmptyValue):
(WTF::IntegralMarkableTraits::emptyValue): IntegralMarkableTraits can be used as an MarkableTraits for integral
types including int etc.
(WTF::Markable::Markable):
(WTF::Markable::operator bool const):
(WTF::Markable::reset):
(WTF::Markable::value const):
(WTF::Markable::value):
(WTF::Markable::operator-> const):
(WTF::Markable::operator->):
(WTF::Markable::operator* const):
(WTF::Markable::operator*):
(WTF::Markable::operator std::optional<T>):
(WTF::Markable::operator std::optional<T> const): This operator allows us to cast Markable<T> to
std::optional<T>.
* wtf/MonotonicTime.h:
(WTF::MonotonicTime::MarkableTraits::isEmptyValue):
(WTF::MonotonicTime::MarkableTraits::emptyValue): MarkableTraits for MonotonicTime. MonotonicTime::nan() is used
as an empty value.
* wtf/Seconds.h:
(WTF::Seconds::MarkableTraits::isEmptyValue):
(WTF::Seconds::MarkableTraits::emptyValue): MarkableTraits for Seconds. Seconds::nan() is used as an empty value.
* wtf/WallTime.h:
(WTF::WallTime::nan):
(WTF::WallTime::MarkableTraits::isEmptyValue):
(WTF::WallTime::MarkableTraits::emptyValue): MarkableTraits for WallTime. WallTime::nan() is used as an empty value.

Tools:

Add tests for Markable.

* TestWebKitAPI/CMakeLists.txt:
* TestWebKitAPI/TestWebKitAPI.xcodeproj/project.pbxproj:
* TestWebKitAPI/Tests/WTF/Markable.cpp: Added.
(TestWebKitAPI::TEST):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@235852 268f45cc-cd09-0410-ab3c-d52691b4dbfc