Simplify placeholders for amp-story MediaPool

[newmuis]

Currently, we use the original video elements in the DOM as placeholders that are then swapped out when the page containing the media is (pre)loaded. We could, instead, use a simpler element (e.g. an image for videos, or a div for audio) to save resources and have fewer media elements in the page.

This would also prevent the browser from trying to keep placeholder media in memory, as only media allocated by MediaPool should consume substantial resources.

[newmuis]

We can do this swap when we register the element; all information needed to play the media should be present at that time.

[newmuis]

I think this may be able to be used to solve #12728 and #12878, so I'll begin investigating.

[ampprojectbot]

This issue hasn't been updated in awhile. @newmuis Do you have any updates?

[newmuis]

This is another bug that is the same root cause as #12728, so I am going to merge it in and make that the master issue.

Duplicate of #12728

[rustycourses]

I am still getting an error form this with stories that have more than 15/16 video clips.

The error appears in the console when you get to the 15/16 video clip: Cannot read property 'classList' of undefined_