title: Chrome-extension: Invoke page script by injection
body: <p>Is it possible to inject a javascript file into the DOM and immediately execute it ?
  I wish to invoke javascript functions within the page/DOM. A single content script will not work because of the isolated worlds. A background page is required to use <code>chrome.tabs.executeScript()</code>.</p>
  
  <p><strong>Simple Example:</strong><br>
  DOM javascript</p>
  
  <pre><code>function sayHello(){
    alert('Hello World');
  }
  </code></pre>
  
  <p>Javascript file to inject</p>
  
  <pre><code>console.log('Injection complete. Now calling DOM script.');
  sayHello();
  </code></pre>