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

mojo/public/cpp/bindings/struct_ptr.h

 // Copyright 2014 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.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_STRUCT_PTR_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_STRUCT_PTR_H_
 
 #include <cstddef>
 #include <memory>
 #include <new>
@@ skipping 87 matching lines @@
 
   MOJO_MOVE_ONLY_TYPE(StructPtr);
 };
 
 // Designed to be used when Struct is small and copyable. Unions are always
 // InlinedStructPtr in practice.
 template <typename Struct>
 class InlinedStructPtr {
  public:
   InlinedStructPtr() : is_null_(true) {}
-  InlinedStructPtr(decltype(nullptr)) : is_null_(true) {}
+  InlinedStructPtr(std::nullptr_t) : is_null_(true) {}
 
   ~InlinedStructPtr() {}
 
-  InlinedStructPtr& operator=(decltype(nullptr)) {
+  InlinedStructPtr& operator=(std::nullptr_t) {
     reset();
     return *this;
   }
 
   InlinedStructPtr(InlinedStructPtr&& other) : is_null_(true) { Take(&other); }
   InlinedStructPtr& operator=(InlinedStructPtr&& other) {
     Take(&other);
     return *this;
   }
 
@@ skipping 48 matching lines @@
 
   mutable Struct value_;
   bool is_null_;
 
   MOJO_MOVE_ONLY_TYPE(InlinedStructPtr);
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_STRUCT_PTR_H_