Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/wtf/PassRefPtr.h

Index: Source/wtf/PassRefPtr.h
diff --git a/Source/wtf/PassRefPtr.h b/Source/wtf/PassRefPtr.h
index 60aface678ec47a2f41a79971bf1b7ee3cb2eac3..828a5ef9cafc4fab8c8fa51550539ded5345f652 100644
--- a/Source/wtf/PassRefPtr.h
+++ b/Source/wtf/PassRefPtr.h
@@ -58,8 +58,10 @@ namespace WTF {
     }
 
     template<typename T> class PassRefPtr {
+        WTF_DISALLOW_CONSTRUCTION_FROM_ZERO(PassRefPtr);
     public:
         PassRefPtr() : m_ptr(0) { }
+        PassRefPtr(std::nullptr_t) : m_ptr(0) { }
         PassRefPtr(T* ptr) : m_ptr(ptr) { refIfNotNull(ptr); }
         template<typename U> PassRefPtr(const RawPtr<U>& ptr, EnsurePtrConvertibleArgDecl(U, T)) : m_ptr(ptr.get()) { refIfNotNull(m_ptr); }
         explicit PassRefPtr(T& ptr) : m_ptr(&ptr) { m_ptr->ref(); }