title: Can web worker be killed off by browser silently ? (pdf.js issue)
body: <p>Can web worker be killed off by browser silently?</p>
  
  <p>The following happen in mobile/desktop safari and mobile chrome in iOS</p>
  
  <p>When using pdf.js to render multiple pdfs (it use web worker to download and render the pdf), some web worker gone missing randomly.
  It happen randomly and , when happen, it happen to different pdf.</p>
  
  <p>Also, when debug in code(we post message back to main thread for debug), found that web worker stop working at different point in code.</p>
  
  <p>This is so hard to track down the problem because </p>
  
  <ol>
  <li>it fail silently without any error</li>
  <li>in safari, there is no any development tools to check web worker status. Unlike chrome, there is the "thread" section to see the threads being used.</li>
  <li>web worker api do not have any close/terminate event for us to log when it is being killed</li>
  </ol>
  
  <p>Any advice would be great!</p>