Make WebPrivatePtr capable of wrapping garbage collected objects.

Source/web/WebDOMMediaStreamTrack.cpp

 /*
  * Copyright (C) 2013 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 34 matching lines @@
 {
     if (WebCore::V8MediaStreamTrack::hasInstance(value, v8::Isolate::GetCurrent())) {
         v8::Handle<v8::Object> object = v8::Handle<v8::Object>::Cast(value);
         return WebDOMMediaStreamTrack(WebCore::V8MediaStreamTrack::toNative(object));
     }
     return WebDOMMediaStreamTrack(0);
 }
 
 void WebDOMMediaStreamTrack::reset()
 {
-    m_private = 0;
+    // FIXME: replace reset() with nullptr assignment (cf. https://codereview.chromium.org/170603003/ )
+    m_private.reset();
 }
 
 void WebDOMMediaStreamTrack::assign(const WebDOMMediaStreamTrack& b)
 {
     m_private = b.m_private;
 }
 
 WebMediaStreamTrack WebDOMMediaStreamTrack::component() const
 {
     return WebMediaStreamTrack(m_private->component());
 }
 
 } // namespace blink