Bug 951616 - Use docshell to get manifest url. r=ochameau

toolkit/devtools/server/actors/webapps.js

@@ -947,8 +947,7 @@ WebappsActor.prototype = {
     let manifestURL;
     switch(event.type) {
       case "appwillopen":
-        let frame = event.target;
-        manifestURL = frame.getAttribute("mozapp")
+        manifestURL = event.detail.manifestURL;
 
         // Ignore the event if we already received an appwillopen for this app
         // (appwillopen is also fired when the app has been moved to background