[PM] Remove TODO about filtering out late lifecycle state changes.

As documented at
https://cs.chromium.org/chromium/src/components/performance_manager/graph/frame_node_impl.h?l=215&rcl=2b413514ddc9679893b20235eb2cecbbca4d8245,
lifecycle state of a frame/page is updated independently from
navigations. There is no need to filter out updates generated prior
to the last navigation.

Bug: 857472
Change-Id: Ifd64c437349ed218847a6ca4486b8cd0248fb375
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869473
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Auto-Submit: François Doray <fdoray@chromium.org>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707813}

chrome/browser/resource_coordinator/tab_lifecycle_unit_source.cc

@@ -407,9 +407,9 @@ void TabLifecycleUnitSource::OnLifecycleStateChanged(
     performance_manager::mojom::LifecycleState state) {
   TabLifecycleUnit* lifecycle_unit = GetTabLifecycleUnit(web_contents);
 
-  // Some WebContents aren't attached to a tab, so there is no corresponding
-  // TabLifecycleUnit.
-  // TODO(fdoray): This may want to filter for the navigation_id.
+  // Lifecycle state is updated independently from navigations. Therefore, there
+  // is no need to filter out the event if it was generated before the last
+  // navigation.
   if (lifecycle_unit)
     lifecycle_unit->UpdateLifecycleState(state);
 }