Autoplay shouldn't block user initialized video

[darkdh]

Test plan

1. Go to https://blog.google/products/photos/google-photos-500-million-new-sharing/
2. Try to play the main video
3. It should play; before it would spin until you accept via the auto-play alert bar

Original issue description

#8739 (comment)

go here: https://blog.google/products/photos/google-photos-500-million-new-sharing/

Then try to play the main video. It spins until you accept the auto-play alert bar.

This is not how it should work. This video should play when you press the play button regardless of how autoplay is set for this page.

Must fix or pull. : (

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#8739 (comment)

from @bradleyrichter, regarding https://boingboing.net/2017/06/21/brave-orfox-firefox-focus.html

We need to improve the auto-play blocking. The top video on this page does not seem to be set to autoplay but we make it hard for the user to play the video which won’t play until you accept the alert.