Added XW_Extension_Permissions.h and XW_Extension_Runtime.h

Added headers for two new public APIs for external extensions.

Runtime is used for querying special runtime data using a key/data pair
interface. For now, only the "app_id" is available.

Permissions allows the extension to check if certain capabilities are
available.

https://crosswalk-project.org/jira/browse/XWALK-1035