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/compositing/compositor.rs

@@ -714,6 +714,18 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 let _ = sender.send(());
             }
 
+            (Msg::GetScrollOffset(pipeline_id, layer_id, sender), ShutdownState::NotShuttingDown) => {
+                match self.find_layer_with_pipeline_and_layer_id(pipeline_id, layer_id) {
+                    Some(ref layer) => {
+                        let typed = layer.extra_data.borrow().scroll_offset;
+                        let _ = sender.send(Point2D::new(typed.x.get(), typed.y.get()));
+                    },
+                    None => {
+                        warn!("Can't find requested layer in handling Msg::GetScrollOffset");
+                    },
+                }
+            }
+
             // When we are shutting_down, we need to avoid performing operations
             // such as Paint that may crash because we have begun tearing down
             // the rest of our resources.

components/compositing/compositor_thread.rs

@@ -98,6 +98,10 @@ pub fn run_script_listener_thread(compositor_proxy: Box<CompositorProxy + 'stati
                 compositor_proxy.send(Msg::TouchEventProcessed(result))
             }
 
+            ScriptToCompositorMsg::GetScrollOffset(pid, lid, send) => {
+                compositor_proxy.send(Msg::GetScrollOffset(pid, lid, send));
+            }
+
             ScriptToCompositorMsg::Exited => break,
         }
     }
@@ -231,6 +235,8 @@ pub enum Msg {
     MoveTo(Point2D<i32>),
     /// Resize the window to size
     ResizeTo(Size2D<u32>),
+    /// Get scroll offset of a layer
+    GetScrollOffset(PipelineId, LayerId, IpcSender<Point2D<f32>>),
     /// A pipeline was shut down.
     // This message acts as a synchronization point between the constellation,
     // when it shuts down a pipeline, to the compositor; when the compositor
@@ -272,6 +278,7 @@ impl Debug for Msg {
             Msg::MoveTo(..) => write!(f, "MoveTo"),
             Msg::ResizeTo(..) => write!(f, "ResizeTo"),
             Msg::PipelineExited(..) => write!(f, "PipelineExited"),
+            Msg::GetScrollOffset(..) => write!(f, "GetScrollOffset"),
         }
     }
 }