Bug 1401236 - Don't reread already loaded data in `JSONFile#load`. r=Gijs

MozReview-Commit-ID: AHD5G453GHM

--HG--
extra : rebase_source : a3787ad99b167c4ff38e82540e3e36d7d8d28734

toolkit/modules/JSONFile.jsm

@@ -178,6 +178,10 @@ JSONFile.prototype = {
    *          if there is no dataPostProcessor.
    */
   async load() {
+    if (this.dataReady) {
+      return;
+    }
+
     let data = {};
 
     try {