Validation errors reported for invalid iframe attributes mozallowfullscreen and webkitallowfullscreen

[westonruter]

Bug Description

Adding an iframe that contains mozallowfullscreen or webkitallowfullscreen attributes results in AMP validation errors being reported for those attributes being invalid. The AMP_Iframe_Sanitizer should just omit those entirely since amp-iframe will add them if needed if the allowfullscreen attribute is present. Example markup:

<iframe title="Why Backbone" id="talk_frame_48643" src="//speakerdeck.com/player/4648d440a3230130452522b217532879" width="640" height="480" style="border:0; padding:0; margin:0; background:transparent;" frameborder="0" allowtransparency="true" allowfullscreen="allowfullscreen" mozallowfullscreen="true" webkitallowfullscreen="true"></iframe>

Expected Behaviour

No validation errors should occur.

Steps to reproduce

1. Add a new Embed block.
2. Supply the URL https://speakerdeck.com/wallat/why-backbone
3. Save draft
4. Notice validation errors

Screenshots

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

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