Add test to constellation to avoid writing reftest image if there are pending frames.

Also change when pipelines become active.

This makes the constellation activate a pipeline as the current frame
when it is ready to do initial reflow, rather than when it is ready
to paint.

This fixes a number of intermittent failures that could previously occur
if an iframe was not visible - which would mean it was never moved from
a pending frame in the constellation to an active frame.

(It happens that webrender exposes these intermittents as permanent failures).

components/msg/constellation_msg.rs

@@ -244,7 +244,6 @@ pub enum MouseButton {
 /// Messages from the paint task to the constellation.
 #[derive(Deserialize, Serialize)]
 pub enum PaintMsg {
-    Ready(PipelineId),
     Failure(Failure),
 }
 
@@ -256,6 +255,15 @@ pub enum AnimationState {
     NoAnimationCallbacksPresent,
 }
 
+/// Used to determine if a script has any pending asynchronous activity.
+#[derive(Copy, Clone, Debug, PartialEq, Deserialize, Serialize)]
+pub enum DocumentState {
+    /// The document has been loaded and is idle.
+    Idle,
+    /// The document is either loading or waiting on an event.
+    Pending,
+}
+
 // https://developer.mozilla.org/en-US/docs/Web/API/Using_the_Browser_API#Events
 #[derive(Deserialize, Serialize)]
 pub enum MozBrowserEvent {