🕵️[story-ads] Heavy ads investigation

As Chrome's [heavy ads intervention](https://developers.google.com/web/updates/2020/05/heavy-ad-interventions) ramps up we should determine the best course of action. The criteria for intervening is as follows:
- Uses the main thread for more than 60 seconds in total
- Uses the main thread for more than 15 seconds in any 30 second window
- Uses more than 4 megabytes of network bandwidth

These appear to be triggering in stories due to the number of autoplaying videos and _possibly_ mis-measurement due to the use of media pool. Some action items
- Investigate these interventions and determine if it is always videos > 4mb or if thread use is triggering
- Sync with partner ads teams to see what steps they are taking
- Potentially monitor if we are going to hit this limit client side and downgrade the video

We also have the option of killing the ad if we near the limit, but I am not sure what we gain as killing the ad is what chrome will do as well.