Bug 1599203 - Set webgl.dxgl.enabled on Windows to true r=njn

This was overlooked when removing duplicates in bug 1575983.

Differential Revision: https://phabricator.services.mozilla.com/D54587

--HG--
extra : moz-landing-system : lando

modules/libpref/init/StaticPrefList.yaml

@@ -7736,7 +7736,11 @@
 
 - name: webgl.dxgl.enabled
   type: RelaxedAtomicBool
+#ifdef XP_WIN
+  value: true
+#else
   value: false
+#endif
   mirror: always
 
 - name: webgl.dxgl.needs-finish