Implement Msg::Status with serialized url upon mouseover

Credits for Mike Blumenkrantz (@zmike), I just rebased against trunk and
fixed the url serialization.

Fixes #6178.

components/compositing/compositor.rs

@@ -420,6 +420,10 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 self.scroll_fragment_to_point(pipeline_id, layer_id, point);
             }
 
+            (Msg::Status(message), ShutdownState::NotShuttingDown) => {
+                self.window.status(message);
+            }
+
             (Msg::LoadStart(back, forward), ShutdownState::NotShuttingDown) => {
                 self.window.load_start(back, forward);
             }

components/compositing/compositor_task.rs

@@ -199,6 +199,8 @@ pub enum Msg {
     ReturnUnusedNativeSurfaces(Vec<NativeSurface>),
     /// Collect memory reports and send them back to the given mem::ReportsChan.
     CollectMemoryReports(mem::ReportsChan),
+    /// A status message to be displayed by the browser chrome.
+    Status(Option<String>),
 }
 
 impl Debug for Msg {
@@ -229,6 +231,7 @@ impl Debug for Msg {
             Msg::HeadParsed => write!(f, "HeadParsed"),
             Msg::ReturnUnusedNativeSurfaces(..) => write!(f, "ReturnUnusedNativeSurfaces"),
             Msg::CollectMemoryReports(..) => write!(f, "CollectMemoryReports"),
+            Msg::Status(..) => write!(f, "Status"),
         }
     }
 }

components/compositing/constellation.rs

@@ -514,6 +514,10 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
                 debug!("constellation got create-WebGL-paint-task message");
                 self.handle_create_webgl_paint_task_msg(&size, attributes, sender)
             }
+            ConstellationMsg::NodeStatus(message) => {
+                debug!("constellation got NodeStatus message");
+                self.compositor_proxy.send(CompositorMsg::Status(message));
+            }
         }
         true
     }

components/compositing/headless.rs

@@ -97,6 +97,7 @@ impl CompositorEventListener for NullCompositor {
             Msg::AssignPaintedBuffers(..) |
             Msg::ChangeRunningAnimationsState(..) |
             Msg::ScrollFragmentPoint(..) |
+            Msg::Status(..) |
             Msg::LoadStart(..) |
             Msg::LoadComplete(..) |
             Msg::ScrollTimeout(..) |

components/compositing/windowing.rs

@@ -107,6 +107,8 @@ pub trait WindowMethods {
     fn set_page_title(&self, title: Option<String>);
     /// Sets the load data for the current page.
     fn set_page_url(&self, url: Url);
+    /// Called when the browser chrome should display a status message.
+    fn status(&self, Option<String>);
     /// Called when the browser has started loading a frame.
     fn load_start(&self, back: bool, forward: bool);
     /// Called when the browser is done loading a frame.