Use window singleton API in the App Developer Tool and simplify the code.

chrome/browser/resources/apps_debugger/background.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 var mainWindow = null;
 chrome.app.runtime.onLaunched.addListener(function() {
-  if (mainWindow && !mainWindow.contentWindow.closed) {
-    mainWindow.focus();
-  } else {
-    chrome.app.window.create('main.html', {
-      id: 'apps_debugger',
-      minHeight: 600,
-      minWidth: 800,
-      height: 600,
-      width: 800,
-    }, function(win) {
-      mainWindow = win;
-    });
-  }
+  chrome.app.window.create('main.html', {
+    id: 'apps_devtool',
+    minHeight: 600,
+    minWidth: 800,
+    height: 600,
+    width: 800,
+    singleton: true
+  });
 });