AMP4Email: Support scrollTo action

[lichinshao]

What's the issue?

AMP email: scrollTo functionality doesn't work on AMP email playground or in gmail client when you send a test email from playground.

How do we reproduce the issue?

1. copy code here, which is essentially amp list Remove usages of for-of #2 example in playground with added srollTo top button. https://glitch.com/edit/#!/alert-mica-ticket
2. paste into AMP3Email playground: https://amp.gmail.dev/playground/
3. Observe bug when you scroll to the bottom and click on the "to top" button. Can also send a test email from playground and view bug in gmail client.

Tips on Rendering Bugs

What browsers are affected?

amp4 email playground, gmail client.

Which AMP version is affected?

[fstanis]

Unfortunately, scrolling in emails is a hard problem (this is a big issue for lightbox: #23170). The email is always fully in viewport, since it's rendered inside an iframe, so you'd need to scroll the parent page which is on a different origin.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.