title: Only allow one active instance of a Chrome extension
body: <p>I have a Chrome extension which needs to be open only in one window per machine at a time.
  What would be the best way to enforce this condition? For example, is there a mechanism to point the user to an existing tab running the extension, if there exists such a tab?</p>
  
  <p>The relevant parts of my manifest file are as follows:</p>
  
  <p><strong>manifest.json</strong></p>
  
  <pre><code>{
  
      "manifest_version": 2,
  
      "browser_action": {
          "default_icon": "/img/favicon.ico",
          "popup": "main.html"
      },
  
      "background": {
        "scripts": ["open.js"]
      }
  
  }
  </code></pre>
  
  <p>And the <code>open.js</code> reads as follows:</p>
  
  <p><strong>open.js</strong></p>
  
  <pre><code>chrome.browserAction.onClicked.addListener(function(tab) {
    chrome.tabs.create({'url': chrome.extension.getURL('test.html')}, function(tab) {
    });
  });
  </code></pre>