Auto merge of #9968 - izgzhen:scroll, r=asajeffrey

Implement scroll, scrollLeft, scrollTop and friends, addressing issue #9650

This is a work in progress to solve https://github.com/servo/servo/issues/9650. Thanks a lot for helping the review.

- [x] scroll
- [x] scrollTo
- [x] scrollBy
- [x] scrollTop (setter and getter)
- [x] scrollLeft (setter and getter)

The setters will be implemented in another PR after this is merged.

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components/script_traits/lib.rs

@@ -376,6 +376,8 @@ pub enum ScriptToCompositorMsg {
     ResizeTo(Size2D<u32>),
     /// Script has handled a touch event, and either prevented or allowed default actions.
     TouchEventProcessed(EventResult),
+    /// Get Scroll Offset
+    GetScrollOffset(PipelineId, LayerId, IpcSender<Point2D<f32>>),
     /// Requests that the compositor shut down.
     Exit,
     /// Allow the compositor to free script-specific resources.