title: How to call functions in the original page(tab) in Chrome Extensions
body: <p>I'm now making a Chrome Extension. I want to call JS functions that are defined in the original page (tab), from Chrome Extension. It doesn't matter whether <code>background.html</code> or <code>Content_Script</code> calls them.</p>
  
  <p>For example:</p>
  
  <p>Original page (tab)</p>
  
  <pre><code>&lt;html&gt;
  &lt;head&gt;
  &lt;title&gt;Original Page&lt;/title&gt;
  &lt;script&gt;
  function greeting(){
      alert("Ohayou!");
      // some other codes here
  }
  &lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;&lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>Then I want to call the function "greeting" in the original page, from Google Extensions.
  How can I do the above?</p>