Add BackgroundSync idl file

interfaces/BackgroundSync.idl

@@ -0,0 +1,24 @@
+partial interface ServiceWorkerRegistration {
+  readonly attribute SyncManager sync;
+};
+
+[Exposed=(Window,Worker)]
+interface SyncManager {
+  Promise<void> register(DOMString tag);
+  Promise<sequence<DOMString>> getTags();
+};
+
+partial interface ServiceWorkerGlobalScope {
+  attribute EventHandler onsync;
+};
+
+[Constructor(DOMString type, SyncEventInit init), Exposed=ServiceWorker]
+interface SyncEvent : ExtendableEvent {
+  readonly attribute DOMString tag;
+  readonly attribute boolean lastChance;
+};
+
+dictionary SyncEventInit : ExtendableEventInit {
+  required DOMString tag;
+  boolean lastChance = false;
+};