Add IDL and initial Blink API for Background Sync

Source/modules/background_sync/SyncEventInit.idl

Index: Source/modules/background_sync/SyncEventInit.idl
diff --git a/Source/core/html/canvas/WebGLSync.idl b/Source/modules/background_sync/SyncEventInit.idl
similarity index 65%
copy from Source/core/html/canvas/WebGLSync.idl
copy to Source/modules/background_sync/SyncEventInit.idl
index a401e20aa857447b84d5b6878d452cafca642fe5..1ab9d737c8300b4832264ca8e073ca93f88c2d24 100644
--- a/Source/core/html/canvas/WebGLSync.idl
+++ b/Source/modules/background_sync/SyncEventInit.idl
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-[
-    RuntimeEnabled=UnsafeES3APIs,
-    WillBeGarbageCollected
-] interface WebGLSync {
-};
+dictionary SyncEventInit : ExtendableEventInit {
+    SyncRegistration registration;
+};