browser-plugin: Accept drags through the new WebPlugin interface

content/test/data/browser_plugin_dragging.html

 <textarea id='id_message'>
 </textarea>
 
 <object id="plugin"
     tabindex="0"
     type="application/browser-plugin"
     width="240"
     height="120"
     style='border: solid 1px red'
-    contentEditable
     >
 </object>
 
 <script>
 function dragLocation() {
   return [id_message.offsetLeft + id_message.offsetWidth / 2,
           id_message.offsetTop + id_message.offsetHeight / 2];
 }
 
 function dropLocation() {
   return [plugin.offsetLeft + plugin.offsetWidth / 2,
           plugin.offsetTop + plugin.offsetHeight / 2];
 }
 
 function SetSrc(src) {
   plugin = document.getElementById('plugin');
   plugin.src = src;
 }
 </script>