Add IDL and initial Blink API for Background Sync

Source/modules/serviceworkers/ServiceWorkerGlobalScope.idl

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 33 matching lines @@
   [CallWith=ScriptState, RaisesException] Promise<Response> fetch(RequestInfo input, optional Dictionary init);
 
   [RaisesException] void close();
 
   [CallWith=ScriptState] Promise<void> skipWaiting();
 
   attribute EventHandler onactivate;
   [RuntimeEnabled=ServiceWorkerOnFetch] attribute EventHandler onfetch;
   attribute EventHandler oninstall;
   attribute EventHandler onmessage;
-  [RuntimeEnabled=BackgroundSync] attribute EventHandler onsync;
 };