These were moved in 0f04c6d but the namespace remained the same. We also
now forward declare these in `AK/Forward.h` rather than
`LibWeb/Forward.h` (or not at all in the case of `ValueComparingRefPtr`)
I originally wanted to move this to prevent a circular dependency, in a
commit that I'm probably not submitting any more. But also, there's
nothing about this type that is CSS or LibWeb-specific, so let's put
this in AK.