Chrome prototype: work around "save visible" captureVisibleTab delay

[jaredhirsch]

On Chrome, the "save visible" captured image sometimes includes the selection overlay. The problem is more pronounced on heavyweight pages, like The Verge.

So far, I've tried:

• switching from toggling display to toggling visibility (helped but not enough)
• toggling visibility and accessing DOM properties of the iframe that should cause a DOM refresh (e.g., checking its offsetWidth) and then checking the visibility is 'hidden' before taking a shot
• toggling visibility, waiting for requestAnimationFrame or a setTimeout inside a Promise, then checking the DOM state and resolving the Promise only when the DOM says the iframe is hidden

It seems there are competing rendering asynchronicities at work, but I'm not sure we have anything that can tell us when the iframe is gone from the DOM.

One option might be to explore the Intersection observer API.

Another option: screenshot the page before displaying the preselection iframe, though this might introduce its own issues.

[jaredhirsch]

Refs #3998

[jaredhirsch]

Ian's working on this, too