Omit mozallowfullscreen & webkitallowfullscreen when sanitizing iframe

[pierlon]

Summary

Omit the mozallowfullscreen & webkitallowfullscreen attributes when sanitizing an iframe.

Fixes #3425.

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[westonruter]

Looks good. Would you add one unit test to AMP_Iframe_Sanitizer using the test code:

<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>

To ensure it gets converted without those attributes as expected?

[pierlon]

Done!