Rectify our use of std::nullptr_t, inclusions of <stddef.h>/<cstddef>.

mojo/edk/util/make_unique.h

 // Copyright 2015 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.
 
 // This is almost the source from N3656 ("make_unique (Revision 1)";
 // https://isocpp.org/files/papers/N3656.txt) almost verbatim, so that we have a
 // "make_unique" to use until we can use C++14. The following changes have been
 // made:
 //   - It's called |MakeUnique| instead of |make_unique|.
 //   - It's in the |mojo::util| namespace instead of |std|; this also
 //     necessitates adding some |std::|s.
 //   - It's been formatted.
 
 #ifndef MOJO_EDK_UTIL_MAKE_UNIQUE_H_
 #define MOJO_EDK_UTIL_MAKE_UNIQUE_H_
 
-#include <cstddef>
+#include <stddef.h>
+
 #include <memory>
 #include <type_traits>
 #include <utility>
 
 namespace mojo {
 namespace util {
 
 template <class T>
 struct _Unique_if {
   typedef std::unique_ptr<T> _Single_object;
@@ skipping 20 matching lines @@
   return std::unique_ptr<T>(new U[n]());
 }
 
 template <class T, class... Args>
 typename _Unique_if<T>::_Known_bound MakeUnique(Args&&...) = delete;
 
 }  // namespace util
 }  // namespace mojo
 
 #endif  // MOJO_EDK_UTIL_MAKE_UNIQUE_H_