diff --git a/components/devtools_traits/lib.rs b/components/devtools_traits/lib.rs
index 4edf19d6f02..b7711821112 100644
--- a/components/devtools_traits/lib.rs
+++ b/components/devtools_traits/lib.rs
@@ -145,7 +145,7 @@ pub struct TimelineMarker {
     pub end_stack: Option<Vec<()>>,
 }
 
-#[derive(PartialEq, Eq, Hash, Clone, Deserialize, Serialize)]
+#[derive(PartialEq, Eq, Hash, Clone, Deserialize, Serialize, HeapSizeOf)]
 pub enum TimelineMarkerType {
     Reflow,
     DOMEvent,
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index 30d51a31c4c..10d47f161be 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -195,7 +195,6 @@ pub struct Window {
     devtools_chan: Option<IpcSender<ScriptToDevtoolsControlMsg>>,
     /// For sending timeline markers. Will be ignored if
     /// no devtools server
-    #[ignore_heap_size_of = "TODO(#6909) need to measure HashSet"]
     devtools_markers: DOMRefCell<HashSet<TimelineMarkerType>>,
     #[ignore_heap_size_of = "channels are hard"]
     devtools_marker_sender: DOMRefCell<Option<IpcSender<TimelineMarker>>>,