Auto merge of #21232 - servo:jdm-patch-22, r=<try>

Enable MutationObserver by default.

All of the WebIDL APIs are implemented, so we might as well have it enabled by default now. I've filed #21233 for the remaining test failures.

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resources/prefs.json

@@ -8,7 +8,7 @@
   "dom.gamepad.enabled": false,
   "dom.microdata.testing.enabled": true,
   "dom.mouseevent.which.enabled": false,
-  "dom.mutation_observer.enabled": false,
+  "dom.mutation_observer.enabled": true,
   "dom.permissions.enabled": false,
   "dom.permissions.testing.allowed_in_nonsecure_contexts": false,
   "dom.serviceworker.timeout_seconds": 60,