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

BUG=242989

Review URL: https://chromiumcodereview.appspot.com/15747009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202839 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/resources/apps_debugger/background.js

@@ -4,17 +4,12 @@
 
 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',
+    id: 'apps_devtool',
     minHeight: 600,
     minWidth: 800,
     height: 600,
     width: 800,
-    }, function(win) {
-      mainWindow = win;
+    singleton: true
   });
-  }
 });