style: [css-nesting] Parse parent selector

This parses the ampersand as a parent selector behind an
(off-by-default) selectors feature.

The plan is to call replace_parent_selector while we're doing the
CascadeData rebuilds, which is where we can have all the ancestor
nesting information.

No behavior change.

Differential Revision: https://phabricator.services.mozilla.com/D167237

components/malloc_size_of/lib.rs

@@ -715,6 +715,7 @@ where
             Component::Root |
             Component::Empty |
             Component::Scope |
+            Component::ParentSelector |
             Component::Nth(..) |
             Component::Host(None) => 0,
         }