If "widget" is already existed in the HashMap when

If "widget" is already existed in the HashMap when
createScriptInstanceForWidget(widget) is called,
we need to release the reference of the matching
npObject to avoid leak.

HTMLPlugInElement::getInstance() only calls
createScriptInstanceForWidget(widget) once. But
HTMLEmbedElement is using its closest ancestor who
has objectTag to get the widget. So the same widget
can exist in the HashMap if both <object> and its
child <embed>'s getInstance() are called.

Fix http://b/issue?id=2553266