GC the RuleTree when the Stylist is dropped.

We only consider doing a GC currently if the root node has a zero refcount.
But that only happens if it has no children -- even weak children keep a
strong reference to their parent.  So at the very least, we should do a
GC specifically when the RuleTree is going away.  (We probably want to add
some other GC opportunities too at some point, otherwise it's easy to
never GC a RuleTree.)

components/style/stylist.rs

@@ -624,6 +624,18 @@ impl Stylist {
     }
 }
 
+impl Drop for Stylist {
+    fn drop(&mut self) {
+        // This is the last chance to GC the rule tree.  If we have dropped all
+        // strong rule node references before the Stylist is dropped, then this
+        // will cause the rule tree to be destroyed correctly.  If we haven't
+        // dropped all strong rule node references before now, then we will
+        // leak them, since there will be no way to call gc() on the rule tree
+        // after this point.
+        unsafe { self.rule_tree.gc(); }
+    }
+}
+
 
 /// Map that contains the CSS rules for a specific PseudoElement
 /// (or lack of PseudoElement).