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

public/platform/WebPrivatePtr.h

 /*
  * Copyright (C) 2010 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 206 matching lines @@
     }
 
     void reset() { storage().release(); }
 
     WebPrivatePtr<T>& operator=(const WebPrivatePtr<T>& other)
     {
         storage().assign(other.storage());
         return *this;
     }
 
-    WebPrivatePtr<T>& operator=(std::nullptr_t&)
-    {
-        reset();
-        return *this;
-    }
-
     template<typename U>
     WebPrivatePtr<T>& operator=(const U& ptr)
     {
         storage().assign(ptr);
         return *this;
     }
 
     T* get() const { return storage().get(); }
 
     T* operator->() const
@@ skipping 19 matching lines @@
     // Disable the copy constructor; classes that contain a WebPrivatePtr
     // should implement their copy constructor using assign().
     WebPrivatePtr(const WebPrivatePtr<T>&);
 
     void* m_storage;
 };
 
 } // namespace blink
 
 #endif