Add poster frame support to amp-youtube

[cramforce]

Youtube has an API for retrieving a poster frame. The URL is like this: http://img.youtube.com/vi/{{ videoId }}/maxresdefault.jpg

If AMP is in prerender mode, we currently don't render YT embeds. We should render the poster frame instead.

You can do that by implementing

/** @override */
    prerenderAllowed() {
      return true;
    }

in amp-youtube and then returning true from layoutCallback if only the poster frame was added. When the user goes to the page, layoutCallback should then be called again (I think :).

[jridgewell]

Related to #1175?

[cramforce]

@jridgewell Yeah, I knew I had it already, but didn't find it :)

[cramforce]

Lets do #1657 first, though. They are very similar.

[mkhatib]

@cramforce what do you mean by "if only the poster frame was added"? Do we expect the users to manually add amp-img for the amp-youtube or do we automatically add that for them?

I've just sent #1718 PR for addressing #1657 - If I am to follow the same logic there I'd probably check if we're in prerender mode and insert an image (or an amp-img) and return from layoutCallback before inserting the iframe. Does that make sense or am I missing something?

[dvoytenko]

I'd assume we can just add one ourselves - no need for "poster" attribute or "amp-img" tag set manually.

[cramforce]

@mkhatib Yes, the logic is the same. The main difference is that we can compute the URL ourselves, so there is no need for an additional attribute.