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

mojo/public/cpp/bindings/struct_ptr.h

Index: mojo/public/cpp/bindings/struct_ptr.h
diff --git a/mojo/public/cpp/bindings/struct_ptr.h b/mojo/public/cpp/bindings/struct_ptr.h
index 1b34a9ec036149c229d63d1f4a139ceb8de96093..66edb89113806fe5b28f85b4c51c159e50ed332c 100644
--- a/mojo/public/cpp/bindings/struct_ptr.h
+++ b/mojo/public/cpp/bindings/struct_ptr.h
@@ -105,11 +105,11 @@ 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;
   }