core(viewport): create ViewportMeta computed artifact

[mattzeunert]

Summary

Get the viewport info from a ViewportMeta computed artifact, instead of the font size and tap target audits calling the viewport audit.

Involves making the three audits involved async because computed artifacts are async.

@brendankenny also mentioned including a notApplicable property in the artifact result, for when Lighthouse is run in desktop mode. I've not done that for now, but will consider it when adding the isMobile artifact or adding notApplicable logic to tap targets.

Related Issues/PRs

Closes #7084

[patrickhulce]

LGTM!

just some naming comments

[brendankenny]

Looks good! My comments are really just style/nits.

The computed artifact needs tests, though. I'd imagine most of the computed/viewport-meta-test.js tests will be what's in viewport-test.js now (plus any other coverage you think we need), so hopefully it'll be painless.

[mattzeunert]

Thanks for the feedback @patrickhulce and @brendankenny!

The computed artifact needs tests, though. I'd imagine most of the computed/viewport-meta-test.js tests will be what's in viewport-test.js now (plus any other coverage you think we need), so hopefully it'll be painless.

Moved most of the tests over to viewport-meta-test, what do you think?

[patrickhulce]

Move most of the tests over to viewport-meta-test, what do you think?

heh, I specifically didn't make the test comment because in refactoring PRs I like to see that the original case and flowthrough works exactly as it used it :)

buuuuuut, since @brendankenny pointed it out, I say yes 👍

[mattzeunert]

Move most of the tests over to viewport-meta-test, what do you think?

Missed a d here, have already made the change.

[mattzeunert]

@brendankenny What do you think of the PR now?

[brendankenny]

LGTM2! 📱🖥🖼📐