If a mouse event is targeting an iframe, forward it to the iframe's inner window

Fixes #8759.

This adds a slow path for cases where the compositor's layer-based hit testing
is incorrect.  To optimize for this case, we could instead replace the
layer hit testing with display-list hit testing done in the paint task.

components/script/dom/htmliframeelement.rs

@@ -185,6 +185,11 @@ impl HTMLIFrameElement {
         self.containing_page_pipeline_id.get()
     }
 
+    #[inline]
+    pub fn pipeline_id(&self) -> Option<PipelineId> {
+        self.pipeline_id.get()
+    }
+
     #[inline]
     pub fn subpage_id(&self) -> Option<SubpageId> {
         self.subpage_id.get()