Define WebDriver integration

[RByers]

We agreed at the F2F meeting yesterday that we should plan to eventually provide WebDriver automation hooks, similar to FedCM and WebAuthn. This will be important for both browser implementation testing and website testing.

In the interim we have some web platform tests which rely on some hacks.

[marcoscaceres]

Ok, yeah... @samuelgoto and I were talking about doing just a simple fake protocol (which we probably need regardless) ... but we should maybe list the requirements for what we might want to emulate through web driver... there might not be that many things:

User actions:

• user accepts
• user aborts

Data related:

• add mock credential

etc...

[marcoscaceres]

Ok, looks like Web Authn might give us a lot of what we need... we can generalize the Web Authn Automation part for this too.

I think we still need the fake protocol though, but we should make it explicit that it's only used "in automation".

[marcoscaceres]

Superseded by #357