More improvements to the google-ima shim script

[kzar]

We have enabled the google-ima shim script again in the DuckDuckGo
Privacy Essentials browser extension, and found a couple more issues:

• Some websites set the enablePreloading[1] option, which should
  cause[2] the AdsManager.init() method to trigger the LOADED AdEvent
  to fire. If the event doesn't fire, those websites can get stuck
  waiting for the event forever.
• When AdsManager.start() method is called, a bunch of events are
  dispatched in order, to simulate ads loading, playing and
  finishing. There was a mistake in that logic though. The
  CONTENT_PAUSE_REQUESTED and CONTENT_RESUME_REQUESTED events[3]
  should fire as the ads start and finish respectively. By firing the
  latter early, and skipping the former, some websites got confused
  and tried to display ad overlays at the same time as playing their
  content, or didn't display they content at all.

1 - https://developers.google.com/interactive-media-ads/docs/sdks/html5/client-side/reference/js/google.ima.AdsRenderingSettings#enablePreloading
2 - https://developers.google.com/interactive-media-ads/docs/sdks/html5/client-side/preload#timing
3 - https://developers.google.com/interactive-media-ads/docs/sdks/html5/client-side/reference/js/google.ima.AdEvent

[kzar]

@gorhill A couple more improvements to the google-ima shim for you. If you'd like to test:

1. Try playing the AI audio version of articles on rawstory.com with the Trinity Audio widget.
2. Try playing a video included with one of the articles on metro.co.uk

(Making sure the google-ima shim is active obviously!)

[gorhill]

Yes, worked. For metro.co.uk, it seems a bit difficult to reproduce but I did get the hung video with spinning wheel once. For rawstory.com, I get the hung player. Incidentally, wholly blocking ima3.js on rawstory.com also solves the issue and it also removes the introduction about sponsor.