Add support for "userContexts" argument to "browsingContext.setViewport" command

[lutien]

Similar to other commands (e.g. session.subscribe) we should add a possibility to set viewport settings to all existing and future browsing contexts belonging to certain userContexts.

[jgraham]

In retrospect it's a bit unfortunate that we put this command in browsingContext. Of course it's only an aesthetic thing and we can easily just add a userContext field, but I wonder if we should have an emulation module with a new version of this command along with the commands for timezone overrides and so on, and deprecate the browsingContext version.

[OrKoN]

I think it might be better to add viewport as an attribute (defaultViewport) for the creation of user contexts unless there is a use case for changing viewport for multiple contexts at once (which I do no see).

[jgraham]

I think we need a coherent plan for how the group of related things (e.g. viewports, geolocation emulation, timezone overrides, preload scripts, event subscriptions) will work.

My concern with having creating a user context allow specifying the viewport configuration is that if we apply the same pattern elsewhere we'll end up with duplication for all the things that we want to configure; one command for targeting a specific context or set of contexts, and one property on user context creation. Or alternatively we'll have a mixed model where some things like viewport overrides are specified when creating a user context, but other things that people want to apply to an entire user context like preload scripts, or event subscriptions, are separate commands.

The pattern we've adopted so far is to avoid single "configure all the things" commands (except capabilities, which I think have turned out to be a generally problematic model beyond the use case of picking a browser instance), and had single commands per configuration option. So my prior is that's the model we should keep in the future.

[OrKoN]

I generally I agree but I also think changing viewport in multiple existing browsing contexts might get somewhat complicated w.r.t. the background tab throttling (I could be overthinking).