Correctly emulate desktop in headless scenarios

[paulirish]

When we're run in headless (LR) and we want the desktop experience, the viewport behavior is quite undefined, leading to screenshots that are cut off:

We think desktop "emulation" is the answer here. I've tried it out and everything looks more expected in headless:

TBH our driver & emulation code don't nicely account for this scenario, so it'll need some thought. @patrickhulce this seems up your alley. watcha thinkin?

[patrickhulce]

yeah I've been chewing on this idea too after #5893, I'm thinking an enum for formFactor or some such name instead of disableMobileEmulation :)

[paulirish]

I'm thinking an enum for formFactor or some such name instead of disableMobileEmulation

that sounds great. disableMobileEmulation is an OK cli flag, but it's proven to be quite cumbersome in core.