iwyu: Cleanup in the following files:

base/callback_internal.h

 // Copyright (c) 2011 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 file contains utility functions and classes that help the
 // implementation, and management of the Callback objects.
 
 #ifndef BASE_CALLBACK_INTERNAL_H_
 #define BASE_CALLBACK_INTERNAL_H_
 #pragma once
 
+#include <stddef.h>
+
 #include "base/base_api.h"
 #include "base/memory/ref_counted.h"
 
 namespace base {
 namespace internal {
 
 // InvokerStorageBase is used to provide an opaque handle that the Callback
 // class can use to represent a function object with bound arguments.  It
 // behaves as an existential type that is used by a corresponding
 // DoInvoke function to perform the function execution.  This allows
@@ skipping 102 matching lines @@
 template <typename T>
 struct ParamTraits<T[]> {
   typedef const T* ForwardType;
   typedef const T* StorageType;
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_CALLBACK_INTERNAL_H_