Add IDL and initial Blink API for Background Sync

Source/modules/background_sync/ServiceWorkerGlobalScopeSync.h

Index: Source/modules/background_sync/ServiceWorkerGlobalScopeSync.h
diff --git a/public/platform/modules/app_banner/WebAppBannerPromptReply.h b/Source/modules/background_sync/ServiceWorkerGlobalScopeSync.h
similarity index 45%
copy from public/platform/modules/app_banner/WebAppBannerPromptReply.h
copy to Source/modules/background_sync/ServiceWorkerGlobalScopeSync.h
index 9781aa600bb62f2aaae53a0be18e6865bae0a2a1..d5dc7374f9a554925e9f7cb61f0dd331e473546b 100644
--- a/public/platform/modules/app_banner/WebAppBannerPromptReply.h
+++ b/Source/modules/background_sync/ServiceWorkerGlobalScopeSync.h
@@ -2,16 +2,18 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef WebAppBannerPromptReply_h
-#define WebAppBannerPromptReply_h
+#ifndef ServiceWorkerGlobalScopeSync_h
+#define ServiceWorkerGlobalScopeSync_h
+
+#include "core/events/EventTarget.h"
 
 namespace blink {
 
-enum class WebAppBannerPromptReply {
-    None = 0,
-    Cancel
+class ServiceWorkerGlobalScopeSync {
+public:
+    DEFINE_STATIC_ATTRIBUTE_EVENT_LISTENER(sync);
 };
 
 } // namespace blink
 
-#endif // WebAppBannerPromptReply_h
+#endif // ServiceWorkerGlobalScopeSync_h