Bug 1366548 - Set dom.enable_performance_observer in server-timing/test_server_timing.html by copying the __dir__.ini from the directory where it should live, so it doesn't fail when we merge to beta

--HG--
rename : testing/web-platform/meta/performance-timeline/__dir__.ini => testing/web-platform/meta/server-timing/__dir__.ini

testing/web-platform/meta/server-timing/__dir__.ini

@@ -0,0 +1 @@
+prefs: [dom.enable_performance_observer:true]