Add V8ValueTraits<std::nullptr_t>.

As nullptr and std::nullptr_t is introduced, it is natural to use it for
ScriptPromiseResolver::resolve, etc. This CL adds V8ValueTraits<std::nullptr_t>
to support that.

BUG=429074

Review URL: https://codereview.chromium.org/691403002

git-svn-id: svn://svn.chromium.org/blink/trunk@184696 bbb929c8-8fbe-4397-9dbb-9b2b20218538