athena: Expose the list of activities from the ActivityManager.

athena/test/base/activity_lifetime_tracker.cc

Index: athena/test/base/activity_lifetime_tracker.cc
diff --git a/athena/test/base/activity_lifetime_tracker.cc b/athena/test/base/activity_lifetime_tracker.cc
index e5aeea5f99a574809d1bf04d142f77564ea9be4f..ea62e4b9a2a0c59a5768a723edb20ea29d90e394 100644
--- a/athena/test/base/activity_lifetime_tracker.cc
+++ b/athena/test/base/activity_lifetime_tracker.cc
@@ -25,6 +25,9 @@ void ActivityLifetimeTracker::OnActivityEnding(Activity* activity) {
   deleted_activity_ = activity;
 }
 
+void ActivityLifetimeTracker::OnActivityOrderChanged() {
+}
+
 Activity* ActivityLifetimeTracker::GetNewActivityAndReset() {
   Activity* activity = new_activity_;
   new_activity_ = nullptr;