add methods to ensure frame state stays consistent

components/constellation/frame.rs

@@ -76,6 +76,13 @@ impl Frame {
     pub fn remove_forward_entries(&mut self) -> Vec<FrameState> {
         replace(&mut self.next, vec!())
     }
+
+    /// Update the current entry of the Frame from an entry that has been traversed to.
+    pub fn update_current(&mut self, pipeline_id: PipelineId, entry: &FrameState) {
+        self.pipeline_id = pipeline_id;
+        self.instant = entry.instant;
+        self.url = entry.url.clone();
+    }
 }
 
 /// An entry in a frame's session history.
@@ -99,6 +106,15 @@ pub struct FrameState {
     pub frame_id: FrameId,
 }
 
+impl FrameState {
+    /// Updates the entry's pipeline and url. This is used when navigating with replacement
+    /// enabled.
+    pub fn replace_pipeline(&mut self, pipeline_id: PipelineId, url: ServoUrl) {
+        self.pipeline_id = Some(pipeline_id);
+        self.url = url;
+    }
+}
+
 /// Represents a pending change in the frame tree, that will be applied
 /// once the new pipeline has loaded and completed initial layout / paint.
 pub struct FrameChange {