add a user-provided constructor to emulated c++11 NullPtr

Source/wtf/NullPtr.h

Index: Source/wtf/NullPtr.h
diff --git a/Source/wtf/NullPtr.h b/Source/wtf/NullPtr.h
index b6b34e444160a8fb0054ef30f705a6ab7d7d534d..c884e9ec1b42aa34efa7d483b5b44ea1a980e45e 100644
--- a/Source/wtf/NullPtr.h
+++ b/Source/wtf/NullPtr.h
@@ -49,6 +49,12 @@ typedef decltype(nullptr) nullptr_t;
 namespace std {
 class nullptr_t {
 public:
+    // Required in order to create const nullptr_t objects without an
+    // explicit initializer, a la:

[Mikhail, 2014/06/10 07:15:24]

As I see at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42844 there is a
condition that "the object is of (possibly cv-qualified) non-POD class type",
nullptr_t should not be the case, right?
This is more like a GCC 4.5 particularity, could you pls add to the comment that
this is actually a build fix for GCC 4.5?

[Mostyn Bramley-Moore, 2014/06/10 07:33:58]

I think the second clause matches here (not the first that you quoted):
"if the object is of const-qualified type, the underlying class type shall have
a user-declared default constructor."
Done.

+    //
+    // const std::nullptr_t nullptr;
+    nullptr_t() { }
+
     // Make nullptr convertible to any pointer type.
     template<typename T> operator T*() const { return 0; }
     // Make nullptr convertible to any member pointer type.