Intent to deprecate and remove: offset-* logical properties.

[Emilio Cobos Álvarez]

In bug 1464782 I renamed these to inset-*, and left the offset-*
properties as an alias behind a (enabled-by-default) pref:

layout.css.offset-logical-properties.enabled

I want it to turn it off by default sooner rather than later, given no
other engine supports it, but keep the pref around for at least a
release, following David's advice in [1].

I just filed bug 1471838 for that.

There's (somewhat unlikely) chance of compat fallout if some page relies
on these, but my understanding is that this is somewhat unlikely given
no other engine supports them yet, and we're at the beginning of a cycle
so toggling the pref back on should be trivial should we need to do so.

-- Emilio

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1464782#c9