Remove unused WebPrivatePtr::operator=(std::nullptr_t).

Source/web/WebIDBKey.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 169 matching lines @@
     m_private = IDBKey::createNumber(number);
 }
 
 void WebIDBKey::assignInvalid()
 {
     m_private = IDBKey::createInvalid();
 }
 
 void WebIDBKey::assignNull()
 {
-    m_private.reset();
+    m_private = nullptr;
 }
 
 void WebIDBKey::reset()
 {
     m_private.reset();
 }
 
 WebIDBKeyType WebIDBKey::keyType() const
 {
     if (!m_private.get())
@@ skipping 45 matching lines @@
     m_private = value;
     return *this;
 }
 
 WebIDBKey::operator PassRefPtr<IDBKey>() const
 {
     return m_private.get();
 }
 
 } // namespace blink