Remove home-brewed std::nullptr_t declaration

SSIA. The standard 'nullptr' is available now on all the
supported platforms.
(See http://chromium-cpp.appspot.com/)

Review URL: https://codereview.chromium.org/601823004

git-svn-id: svn://svn.chromium.org/blink/trunk@183102 bbb929c8-8fbe-4397-9dbb-9b2b20218538