Bug 1789745 - Add usecounter for Prioritized Task Scheduling API, r=sefeng

Differential Revision: https://phabricator.services.mozilla.com/D156803

dom/base/UseCounters.conf

@@ -415,5 +415,7 @@ custom PrivateBrowsingCachesDelete calls caches.delete in Private Browsing Mode
 custom PrivateBrowsingCachesKeys calls caches.keys in Private Browsing Mode
 custom PrivateBrowsingNavigatorServiceWorker accesses navigator.serviceWorker in Private Browsing Mode
 
+method Scheduler.postTask
+
 // NOTE: Adding use counters requires data review, see
 // https://wiki.mozilla.org/Data_Collection

dom/base/UseCountersWorker.conf

@@ -73,3 +73,5 @@ custom PrivateBrowsingCachesHas calls caches.has in Private Browsing Mode
 custom PrivateBrowsingCachesOpen calls caches.open in Private Browsing Mode
 custom PrivateBrowsingCachesDelete calls caches.delete in Private Browsing Mode
 custom PrivateBrowsingCachesKeys calls caches.keys in Private Browsing Mode
+
+method Scheduler.postTask

dom/webidl/WebTaskScheduling.webidl

@@ -22,6 +22,7 @@ callback SchedulerPostTaskCallback = any ();
 
 [Exposed=(Window, Worker), Pref="dom.enable_web_task_scheduling"]
 interface Scheduler {
+  [UseCounter]
   Promise<any> postTask(
     SchedulerPostTaskCallback callback,
     optional SchedulerPostTaskOptions options = {}