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..4656f9efb5efde7e5641c160c0b32b2719eebe63 100644
--- a/Source/wtf/NullPtr.h
+++ b/Source/wtf/NullPtr.h
@@ -49,6 +49,9 @@ typedef decltype(nullptr) nullptr_t;
 namespace std {
 class nullptr_t {
 public:
+    // Required to create uninitialized const nullptr's.

[Peter Kasting, 2014/06/09 23:07:00]

Nit: Do not use an apostrophe to make something plural.

Consider an example of an "uninitialized const nullptr", as I consider myself
reasonably knowledgable about C++ and have trouble parsing that.

[Mostyn Bramley-Moore, 2014/06/09 23:33:30]

Rephrased to avoid the apostrophe, hopefully it's a bit more readable now.

+    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.