<webview>: Partially migrate loadcommit event from content to chrome

content/test/data/browser_plugin_embedder.html

 <script type="text/javascript">
 function sizeChanged(evt) {
   var data = JSON.parse(evt.detail);
   document.title = "AutoSize(" + data.newWidth + ", " + data.newHeight + ")";
 }
 
 function loadAbort(evt) {
   document.title = JSON.parse(evt.detail).reason;
 }
 function loadStart(evt) {
   document.title = JSON.parse(evt.detail).url;
 }
 function loadStop(evt) {
   document.title = "loadStop";
 }
 
-var commitIsTopLevel;
-function loadCommit(evt) {
-  var detail = JSON.parse(evt.detail);
-  document.title = "loadCommit:" + detail.url;
-  commitIsTopLevel = detail.isTopLevel;
-}
-
 var redirectOldUrl;
 var redirectNewUrl;
 function loadRedirect(event) {
   document.title = "redirected";
   var detail = JSON.parse(event.detail);
   if (detail.isTopLevel) {
     redirectOldUrl = detail.oldUrl;
     redirectNewUrl = detail.newUrl;
   }
 }
@@ skipping 66 matching lines @@
       window.document.title = 'main guest';
     }
   }
 }
   var plugin = document.getElementById('plugin');
   plugin.addEventListener('-internal-loadstart', loadStart);
   plugin.addEventListener('-internal-loadabort', loadAbort);
   plugin.addEventListener('-internal-loadredirect', loadRedirect);
   window.addEventListener('message', receiveMessage, false);
   plugin.addEventListener('-internal-loadstop', loadStop);
-  plugin.addEventListener('-internal-loadcommit', loadCommit);
   plugin.addEventListener('-internal-sizechanged', sizeChanged);
 </script>