Interface SetScrollPosition

Indicates two things:

1. The current window (i.e. the old preview iframe) is sending its scroll position to the recipient window (i.e. CMS).
2. The current window (i.e. the CMS) is instructing the recipient window (i.e. the new preview iframe) to set its scroll position to the given coordinates. For simplicity, the same message type is used for both purposes instead of creating separate message types. This allows the CMS to pass over the message received from the old iframe to the new iframe as-is.