Remove constellation round trip for subpage mapping in compositor.

This makes use of the new functionality that allows iframes to generate their own pipeline IDs in order to remove any knowledge of subpage ids from the compositor.

(This is the first of several commits removing subpage from parts of servo).

components/script/dom/htmliframeelement.rs

@@ -222,6 +222,11 @@ impl HTMLIFrameElement {
     pub fn subpage_id(&self) -> Option<SubpageId> {
         self.subpage_id.get()
     }
+
+    #[inline]
+    pub fn pipeline_id(&self) -> Option<PipelineId> {
+        self.pipeline_id.get()
+    }
 }
 
 pub fn Navigate(iframe: &HTMLIFrameElement, direction: NavigationDirection) -> Fallible<()> {