[browsingContext.captureScreenshot/browsingContext.print] when exactly do we take a screenshot? #1131

We have an inline issue in the spec, https://w3c.github.io/webdriver-bidi/#issue-b2b83ca0

This I think means we should be running the steps after the relevant document in:

For each doc of docs, run the animation frame callbacks for doc, passing in the relative high resolution time given frameTimestamp and doc's relevant global object as the timestamp.

This does mean that we don't wait for other documents (e.g., child frames), and maybe more critically don't wait for:

For each doc of docs, update the rendering or user interface of doc and its node navigable to reflect the current state.

Which is the point at which the rendering actually happens.