[scheduler]: Fix tracing issues with nested IdlePeriod trace events.

components/scheduler/child/task_queue_manager.cc

Index: components/scheduler/child/task_queue_manager.cc
diff --git a/components/scheduler/child/task_queue_manager.cc b/components/scheduler/child/task_queue_manager.cc
index 1e6b25ea4f4d3d59c10eace066c63957cc3411e1..33388fbdf8caf142ed3759401ec9a99051ffa044 100644
--- a/components/scheduler/child/task_queue_manager.cc
+++ b/components/scheduler/child/task_queue_manager.cc
@@ -281,6 +281,8 @@ bool TaskQueueManager::ProcessTaskFromWorkQueue(
   if (!pending_task.nestable && main_task_runner_->IsNested()) {
     // Defer non-nestable work to the main task runner.  NOTE these tasks can be
     // arbitrarily delayed so the additional delay should not be a problem.
+    // TODO(skyostil): Figure out a way to not forget which task queue the
+    // task is associated with. See http://crbug.com/522843.
     main_task_runner_->PostNonNestableTask(pending_task.posted_from,
                                            pending_task.task);
   } else {