Add IDL and initial Blink API for Background Sync

Source/modules/background_sync/SyncRegistration.idl

Index: Source/modules/background_sync/SyncRegistration.idl
diff --git a/Source/modules/encryptedmedia/MediaKeyStatusMap.idl b/Source/modules/background_sync/SyncRegistration.idl
similarity index 39%
copy from Source/modules/encryptedmedia/MediaKeyStatusMap.idl
copy to Source/modules/background_sync/SyncRegistration.idl
index f0775839f6bfcf65a1909be51f43114cb5a642c5..c95e1524b1c51152082d9da170d0103ca4a6b8c4 100644
--- a/Source/modules/encryptedmedia/MediaKeyStatusMap.idl
+++ b/Source/modules/background_sync/SyncRegistration.idl
@@ -2,18 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// https://w3c.github.io/encrypted-media/#idl-def-MediaKeyStatusMap
-
-enum MediaKeyStatus {
-    "usable",
-    "expired",
-    "output-not-allowed",
-    "internal-error"
-};
-
 [
+    Exposed=(Window,ServiceWorker),
     GarbageCollected,
-] interface MediaKeyStatusMap {
-    readonly attribute unsigned long size;
-    readonly maplike <BufferSource, MediaKeyStatus>;
+    RuntimeEnabled=BackgroundSync,
+] interface SyncRegistration {
+    readonly attribute DOMString id;
+    readonly attribute SyncRegistrationOptions options;
+    [CallWith=ScriptState] Promise<boolean> unregister();
 };