Bug 1575385: Fix race in browser_jsonview_nojs.js. r=mccr8

Differential Revision: https://phabricator.services.mozilla.com/D42925

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

devtools/client/jsonview/test/browser_jsonview_nojs.js

@@ -6,8 +6,8 @@
 add_task(async function() {
   info("Test JSON without JavaScript started.");
 
-  const oldPref = SpecialPowers.getBoolPref("javascript.enabled");
-  SpecialPowers.setBoolPref("javascript.enabled", false);
+  const oldPref = Services.prefs.getBoolPref("javascript.enabled");
+  Services.prefs.setBoolPref("javascript.enabled", false);
 
   const TEST_JSON_URL = "data:application/json,[1,2,3]";
 
@@ -21,5 +21,5 @@ add_task(async function() {
   );
   is(text, "[1,2,3]", "The raw source should be visible.");
 
-  SpecialPowers.setBoolPref("javascript.enabled", oldPref);
+  Services.prefs.setBoolPref("javascript.enabled", oldPref);
 });