browser-plugin: Remove event handling code, and use CustomEvents in webkit instead.

content/test/data/browser_plugin_embedder.html

 <script type="text/javascript">
 function loadAbort(evt) {
-  document.title = evt.type;
+  document.title = evt.reason;
 }
 function loadStart(evt) {
   document.title = evt.url;
 }
 function loadStop(evt) {
   document.title = "loadStop";
 }
 
 var commitIsTopLevel;
 function loadCommit(evt) {
@@ skipping 81 matching lines @@
   }
 }
   var plugin = document.getElementById('plugin');
   plugin.addEventListener('loadstart', loadStart);
   plugin.addEventListener('loadabort', loadAbort);
   plugin.addEventListener('loadredirect', loadRedirect);
   window.addEventListener('message', receiveMessage, false);
   plugin.addEventListener('loadstop', loadStop);
   plugin.addEventListener('loadcommit', loadCommit);
 </script>