Split paint task messages from ScriptMsg

Refs: https://github.com/servo/servo/issues/8592

components/msg/constellation_msg.rs

@@ -294,7 +294,6 @@ pub enum ScriptMsg {
     NewFavicon(Url),
     /// Status message to be displayed in the chrome, eg. a link URL on mouseover.
     NodeStatus(Option<String>),
-    PainterReady(PipelineId),
     /// Notification that this iframe should be removed.
     RemoveIFrame(PipelineId),
     ScriptLoadedURLInIFrame(IframeLoadInfo),
@@ -306,6 +305,13 @@ pub enum ScriptMsg {
     ViewportConstrained(PipelineId, ViewportConstraints),
 }
 
+/// Messages from the paint task to the constellation.
+#[derive(Deserialize, Serialize)]
+pub enum PaintMsg {
+    Ready(PipelineId),
+    Failure(Failure),
+}
+
 #[derive(Clone, Eq, PartialEq, Deserialize, Serialize, Debug)]
 pub enum AnimationState {
     AnimationsPresent,