style: Propagate flags from original property to aliases

Even we don't have internal aliases right now (and that seems a bit
silly) we do have pref-gated aliases. An alias ID passed to IsEnabled
with the wrong EnabledState would misbehave, assert, and crash.

Though we don't have such callers in the tree because InspectorUtils
passes only arguments that make us not look at the flags, it seems more
reliable this way.

Differential Revision: https://phabricator.services.mozilla.com/D151594

components/style/properties/data.py

@@ -584,6 +584,7 @@ class Alias(object):
         self.gecko_pref = gecko_pref
         self.transitionable = original.transitionable
         self.rule_types_allowed = original.rule_types_allowed
+        self.flags = original.flags
 
     @staticmethod
     def type():