Expose an API to emulate device resolution (devicePixelRatio)

[noamr]

Continuing the discussion in #1378 (comment)
This came up from trying to create web platform tests that take srcset into account.

All browser engines today contain an option to emulate devicePixelRatio.
Several web features rely on this - for example, responsive images.
However, there is no cross-browser API to emulate this value for the purpose of testing.

Suggesting that this would be a parameter when creating a window, or an additional POST (similar to /rect)

[whimboo]

I wonder if we should reserve this for webdriver-bidi only. Or how do we handle feature requests like that for the HTTP-based webdriver protocol? @jgraham?

[gsnedders]

w3c/webdriver-bidi#529 is the corresponding webdriver-bidi feature request.

And it has been added to webdriver-bidi in w3c/webdriver-bidi@af41c9b (w3c/webdriver-bidi#557).