ContentWidth audit doesn't measure content width

[patrickhulce]

The current content-width audit was meant to be modeled after PSI's SizeContentToViewport check, but it fails to cover much of what it does. (Currently, it pretty much just checks that the meta viewport doesn't specify a width that's too small).

Ideally, this would be a full replacement for PSI's version that would check how much of the content requires horizontal scrolling.

[paulirish]

@patrickhulce fixed by #1807 ?

[patrickhulce]

Nope wasn't fixed by that, just noticed there :)