Amphtml: Disable sync-xhr in amp-iframe/amp-ad

[RByers]

What's the issue?

Chrome 65+ allows synchronous XHR to be disabled. Blocking the UI thread on the network is a perf anti-pattern and so should probably not be allowed in AMP documents or embeds.

How do we reproduce the issue?

1. Create an amp page with an amp-iframe to a site doing a sync-xhr
2. View in the top stories carousel
3. Observe jank if you try to swipe when the sync-XHR occurs (especially on a slow network - use throttling in devtools to emulate).

What browsers are affected?

Chrome for now, but at TPAC all other browsers indicated a desire to add this feature as a first step of trying to phase-out sync-xhr.

Which AMP version is affected?

All

/cc @clelland @cramforce with whom this has been discussed offline.

[cramforce]

I think we should launch this for amp-ad and amp-iframe.

@lannka Could you take this on?

[cramforce]

to @jastiv for prioritization.

[jasti]

Given current commitments, it looks like we'll tackle this in the second half of April.
CC @zhouyx

[cramforce]

I'll try to implement on my flight back from Australia. I'll implement behind an experiment and then we can see how we can ship it.

[jasti]

@cramforce did you have a chance to look at this? If not, we can probably reassign to someone on the ads team.

[cramforce]

Turned this on in canary 5 minutes ago in #16524

Implementation in #14481

[cramforce]

Shipping this with our next prod release. Let's see how it goes :)

[cramforce]

Shipped yesterday 🎉