Bug 1220926 - include some sync preferences in about:support. r=bsmedberg

toolkit/modules/Troubleshoot.jsm

@@ -85,6 +85,11 @@ const PREFS_WHITELIST = [
   "print.",
   "privacy.",
   "security.",
+  "services.sync.declinedEngines",
+  "services.sync.lastPing",
+  "services.sync.lastSync",
+  "services.sync.numClients",
+  "services.sync.engine.",
   "social.enabled",
   "storage.vacuum.last.",
   "svg.",