Support blob URLs in <amp-video>

## What's the issue?

I develop CMS with AMP support, it has a feature of amp story preview before publishing it to the server.

User selects local video and it's converted to object url using `URL.createObjectURL()` to be able to preview the result before uploading video to the server. The resulting url is in the following format: `blob:https:///7f6f6c53-6b4b-4282-bcac-bbc67e543d88`.

Then the object url is used as a source of `amp-video` which works but throws errors in console:
`Uncaught Error: amp-video source must start with "https://" or "//" or be relative and served from either https or from localhost. Invalid value: blob:https:///7f6f6c53-6b4b-4282-bcac-bbc67e543d88​​​`.

The same error if object url is used for amp-video.poster.

## What browsers are affected?

All browsers

## Which AMP version is affected?

2004030010070