script: Keep the DOM-side viewport up to date when scrolling happens in

WebRender.

This happens asynchronously, just as it does in non-WebRender mode.

This functionality is a prerequisite for doing proper display-list-based
hit testing in WebRender, since it moves the scroll offsets into Servo
(and, specifically, into the script thread, enabling iframe event
forwarding) instead of keeping them private to WebRender.

Requires servo/webrender_traits#55 and servo/webrender#277.

Partially addresses #11108.

components/script/script_runtime.rs

@@ -62,6 +62,7 @@ pub enum ScriptThreadEventCategory {
     NetworkEvent,
     Resize,
     ScriptEvent,
+    SetScrollState,
     SetViewport,
     StylesheetLoad,
     TimerEvent,