Bug 1618013 - Enable backdrop-filter for early beta r=emilio

Resolution from intent-to-ship discussion is to release for early beta or earlier.

Differential Revision: https://phabricator.services.mozilla.com/D80528

modules/libpref/init/StaticPrefList.yaml

@@ -5672,7 +5672,7 @@
 # Is support for CSS backdrop-filter enabled?
 - name: layout.css.backdrop-filter.enabled
   type: bool
-  value: false
+  value: @IS_EARLY_BETA_OR_EARLIER@
   mirror: always
 
 # Is support for CSS conic-gradient enabled?