Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/wtf/RawPtr.h

Index: Source/wtf/RawPtr.h
diff --git a/Source/wtf/RawPtr.h b/Source/wtf/RawPtr.h
index 439d1c2f3b2c43716764446073ced77cd9c41ec9..5fa5480fc34d2014d55ecc596e1263a3f0dc5a4d 100644
--- a/Source/wtf/RawPtr.h
+++ b/Source/wtf/RawPtr.h
@@ -44,8 +44,11 @@ namespace WTF {
 
 template<typename T>
 class RawPtr {
+    WTF_DISALLOW_CONSTRUCTION_FROM_ZERO(RawPtr);
+    WTF_DISALLOW_ZERO_ASSIGNMENT(RawPtr);
 public:
     RawPtr() : m_ptr(0) { }
+    RawPtr(std::nullptr_t) : m_ptr(0) { }
     RawPtr(T* ptr) : m_ptr(ptr) { }
     RawPtr(const RawPtr& other)
         : m_ptr(other.get())
@@ -87,6 +90,12 @@ public:
         return *this;
     }
 
+    RawPtr& operator=(std::nullptr_t)
+    {
+        m_ptr = 0;
+        return *this;
+    }
+
     operator T*() const { return m_ptr; }
     T& operator*() const { return *m_ptr; }
     T* operator->() const { return m_ptr; }