Support `playsfullscreen` on `amp-video`

Authors should be able to specify where a video plays inline or full-screen when it starts playing.

Safari/iOS has always had good support for this feature but Chrome did not. We should polyfill Chrome.

play-fullscreen should behave as follows in ALL PLATFORMS:

• When user taps the video to play or when the video is played through the play AMP action, it should go to native full-screen (not lightbox or fill)
• If play-fullscreen is used with autoplay, video autoplays inline but when user taps the autoplaying video to unmute it, then it goes to full-screen (alternative it ignoring play-fullscreen when used with autoplay) @abeck000 thoughts?

Stretch goal: Support this attribute on <amp-youtube> and other players. (This feature is already half possible on amp-youtube by setting data-params-playsinline=0 but that only works on iOS)

/cc @rudygalfi

[aghassemi]

closing due to simple workaround documented above, don’t think it is worth polluting the API.

[aghassemi]

Actually since we support fullscreen action. This can already be done.

<amp-video on="tap: video.play, video.fullscreen">

or

<amp-video on="firstPlay: video.fullscreen">