title: Google Chrome Extension: captureVisibleTab problem
body: <p>I'm trying to capture the current visible tab but I'm receiving undefined.
  The following code is executing when the extension's icon is pressed. When the alert is called I see <em>undefined</em> instead of an URL.</p>
  
  <pre><code>chrome.browserAction.onClicked.addListener(function(tab) {            
    chrome.windows.getCurrent(function (win) {    
        chrome.tabs.captureVisibleTab(win.id,{"format":"png"}, function(imgUrl) {
              alert(imgUrl);                                            
        });    
    });    
  });
  </code></pre>
  
  <p>What should I do to get the URL of the captured image?
  Can someone please help me out with this.</p>
  
  <p>Thanks!</p>