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

mojo/edk/util/ref_counted.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.
 
 // Provides a base class for reference-counted classes.
 
 #ifndef MOJO_EDK_UTIL_REF_COUNTED_H_
 #define MOJO_EDK_UTIL_REF_COUNTED_H_
 
-#include <assert.h>
-
-#include <cstddef>
-#include <utility>
-
 #include "mojo/edk/util/ref_counted_internal.h"
 #include "mojo/edk/util/ref_ptr.h"
 #include "mojo/public/cpp/system/macros.h"
 
 namespace mojo {
 namespace util {
 
 // A base class for (thread-safe) reference-counted classes. Use like:
 //
 //   class Foo : public RefCountedThreadSafe<Foo> {
@@ skipping 107 matching lines @@
 // If you want to keep your constructor(s) private and still want to use
 // |MakeRefCounted<T>()|, use this. (See the example above
 // |RefCountedThreadSafe|.)
 #define FRIEND_MAKE_REF_COUNTED(T) \
   friend class ::mojo::util::internal::MakeRefCountedHelper<T>
 
 }  // namespace util
 }  // namespace mojo
 
 #endif  // MOJO_EDK_UTIL_REF_COUNTED_H_