title: chrome.windows undefined for Chrome Extension
body: <p>I'm building a Chrome extension that allows the user to manage open tabs for an application (website). The manifest is:</p>
  
  <pre><code>{
      "manifest_version": 2,
      "name": "AT Tabs",
      "version": "0.1",
      "permissions": ["activeTab", "tabs"],
      "content_scripts": [{
          "matches": ["&lt;all_urls&gt;"],
          "js": ["main.js"]
      }]
  }
  </code></pre>
  
  <p>But when I do this in the main.js file:</p>
  
  <p><code>console.log(chrome.windows);</code></p>
  
  <p>I get undefined in the console... Any ideas why? I have both tabs and activeTab as permissions and the extension is being run in the developer mode.</p>