compositing: Fix a couple of bugs that prevented iframes from painting

after navigation.

The first bug was that iframes were not reflowed in their parent DOM
when the child page navigated. This is fixed by simply having the
constellation notify the appropriate script thread when navigation
occurs.

The second bug was that the compositor was unable to adjust the pipeline
for existing iframe layers, only new ones. This patch adds logic to do
that.

Closes #8081.

components/script_traits/lib.rs

@@ -143,6 +143,8 @@ pub enum ConstellationControlMsg {
         /// The pipeline that contains a frame loading the target pipeline.
         parent: PipelineId
     },
+    /// Notifies a parent frame that one of its child frames is now active.
+    FramedContentChanged(PipelineId, SubpageId),
     /// Report an error from a CSS parser for the given pipeline
     ReportCSSError(PipelineId, String, u32, u32, String),
 }