Fix incorrect Firefox view on webkit scrollbars

[FremyCompany]

See comment in #645

[ExE-Boss]

Firefox is working on this in bug 1432935.

[FremyCompany]

Yes, the current plan is what Xidorn outlined in comment 6. Note that these will be stylesheet fixups that we apply through our web compat system add-on code on incompatible sites.

Comment 6:

My general proposal for fixing this kind of webcompat issue is to create a specific injection stylesheet for each site we want to fix. We may need to wait for the scrollbar properties to ship since it doesn't seem to me that we insert the stylesheets as chrome stylesheets, so we have to wait for the properties to be available in content.

The script was an attempt to create a generalized solution, but there doesn't seem to be enough websites to justify that extra complexity. A specialized stylesheet fixup for each site should be straightforward, and we can also hand such stylesheets to the websites when outreaching to help them fix their sites.

So supporting webkit scrollbars is "not currently planned" (but they could change their mind)