style: Add code to make part rules affect the style of the elements.

I still haven't implemented each_part(), so this will do nothing yet.

The cascade order stuff is fishy, I know, and I'll fix in a followup if it's
fine with you. I moved the sorting of the rules to rule_collector, since it
seemed to me it was better that way that duplicating the code, and those
SelectorMap functions only have a single caller anyway.

Differential Revision: https://phabricator.services.mozilla.com/D32647

components/style/gecko/wrapper.rs

@@ -626,11 +626,6 @@ impl<'le> GeckoElement<'le> {
         }
     }
 
-    #[inline]
-    fn has_part_attr(&self) -> bool {
-        self.as_node().get_bool_flag(nsINode_BooleanFlag::ElementHasPart)
-    }
-
     #[inline]
     fn may_have_anonymous_children(&self) -> bool {
         self.as_node()
@@ -1353,6 +1348,11 @@ impl<'le> TElement for GeckoElement<'le> {
         unsafe { bindings::Gecko_HasAttr(self.0, namespace.0.as_ptr(), attr.as_ptr()) }
     }
 
+    #[inline]
+    fn has_part_attr(&self) -> bool {
+        self.as_node().get_bool_flag(nsINode_BooleanFlag::ElementHasPart)
+    }
+
     // FIXME(emilio): we should probably just return a reference to the Atom.
     #[inline]
     fn id(&self) -> Option<&WeakAtom> {