Refactor the way singleton-style resources are exposed via PPB_Instance

ppapi/shared_impl/singleton_resource_id.h

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef PPAPI_SHARED_IMPL_SINGLETON_RESOURCE_ID_H_
+#define PPAPI_SHARED_IMPL_SINGLETON_RESOURCE_ID_H_
+
+namespace ppapi {
+
+// These IDs are used to access singleton resource objects using
+// PPB_Instance_API.GetSingletonResource.
+enum SingletonResourceID {
+  FLASH_SINGLETON_ID,
+  FLASH_CLIPBOARD_SINGLETON_ID,
+  GAMEPAD_SINGLETON_ID,
+};
+
+}  // namespace ppapi
+
+#endif  // PPAPI_SHARED_IMPL_SINGLETON_RESOURCE_ID_H_