Trigger sequential (and spatial) focus movements

[hugoholgersson]

To be able to test sequential navigation (and also spatial navigation) in WPT, we need a WebDriver method for moving focus: forward, backward (and also up, down, left, right).

We cannot use WebDriver's "send keys"-method because:

1. Browsers might use different key-mappings even though typically tab-key is used.
2. Send keys might do unwanted scrolling.
3. We do not want to target a specific element.

We simply want the browser to move focus in whatever browser-dependent way it likes (and resolve a promise once it has done so).

As of now, Chrome's sequential and spatial navigation tests must use eventSender. This blocks us from upstreaming all these tests to WPT.

[foolip]

I think the remote end steps for an API like this would be very simple, just invoking https://html.spec.whatwg.org/multipage/interaction.html#sequential-focus-navigation.

But curious to hear if there are already robust ways of achieving this.