Add XW_Extension_Permissions.h and XW_Extension_Runtime.h

This defines two internal interfaces in Crosswalk to enforce
permission control for extension API and get varibles for runtime.

Also XW_Extension_SyncMessage.h is updated with that in crosswalk
Merge pull request #253 from xuzhang/extension_update

Add XW_Extension_Permissions.h and XW_Extension_Runtime.h