title: Chrome tabs.onActivated.addListener throws an "undefined" TypeError
body: <p>I'm doing a chrome extension, and there is in the doc <a href="http://code.google.com/chrome/extensions/tabs.html#event-onActivated" rel="noreferrer">this statement</a> about <code>chrome.tabs.onActivated</code>.</p>
  
  <p>Whenever I try to place <code>chrome.tabs.onActivated.addListener</code>, it says <code>Uncaught TypeError: Cannot call method 'addListener' of undefined</code>.</p>
  
  <p>The whole background.html :</p>
  
  <pre><code>&lt;script&gt;
  chrome.tabs.onActivated.addListener(function(info) {
      var tab = chrome.tabs.get(info.tabId, function(tab) {
          localStorage["current_url"] = tab.url;
      });
  });
  &lt;/script&gt;
  </code></pre>