title: workbox: precacheController cleanup error
body: <p>I am setting a precache controller following the <a href="https://developers.google.com/web/tools/workbox/modules/workbox-precaching#advanced_usage" rel="nofollow noreferrer">documentation</a>. It seems to work fine, only for the cleanup part.</p>
  
  <pre><code>// Precache items
  const precacheManifest = 
  [
      {url: offlinePage, revision: '1.0.2'},
      {url: HomePage, revision: '1.0.2'},
      {url: './image/placeholder.png', revision: '1'},    
      {url: './manifest.json', revision: '1.0.1'},        
  ];
  
  const precacheController = new workbox.precaching.PrecacheController(precacheName);
  precacheController.addToCacheList(precacheManifest);
  </code></pre>
  
  <p><strong>Install event</strong> works fine and inserts into cache on install.</p>
  
  <pre><code>/**
   * Install [Event]
   */
  self.addEventListener('install', event =&gt; {
      console.log('install event.');
      event.waitUntil(precacheController.install());
  });
  </code></pre>
  
  <p>The issue is with <strong>activate event</strong> it returns an error in the console: </p>
  
  <blockquote>
    <p>Uncaught TypeError: precacheController.cleanup is not a function.</p>
  </blockquote>
  
  <pre><code>/**
   * Activate [Event]
   */
  self.addEventListener('activate', event =&gt; {
      console.log('activate event.');
      event.waitUntil(precacheController.cleanup());
  });
  </code></pre>
  
  <p>What is wrong with my implementation?
  I am using Workbox 3.6.1</p>