Make the default_app unresizable.

atom/browser/default_app/default_app.js

@@ -17,6 +17,7 @@ app.on('ready', function() {
   mainWindow = new BrowserWindow({
     width: 800,
     height: 600,
+    resizable: false,
     'use-content-size': true,
   });
   mainWindow.loadUrl('file://' + __dirname + '/index.html');