mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/test_change_tracker.cc

Index: components/mus/ws/test_change_tracker.cc
diff --git a/components/mus/ws/test_change_tracker.cc b/components/mus/ws/test_change_tracker.cc
index 7f411e682ce07722d149aa99e55d01fbd16b8d7c..d03cb8e9a3bfcbae42e049e035610b07f0976f9f 100644
--- a/components/mus/ws/test_change_tracker.cc
+++ b/components/mus/ws/test_change_tracker.cc
@@ -114,6 +114,10 @@ std::string ChangeToDescription(const Change& change,
                                 WindowIdToString(change.window_id).c_str(),
                                 change.event_action);
 
+    case CHANGE_TYPE_EVENT_OBSERVED:
+      return base::StringPrintf("EventObserved event_action=%d",
+                                change.event_action);
+
     case CHANGE_TYPE_PROPERTY_CHANGED:
       return base::StringPrintf("PropertyChanged window=%s key=%s value=%s",
                                 WindowIdToString(change.window_id).c_str(),
@@ -370,6 +374,13 @@ void TestChangeTracker::OnWindowInputEvent(Id window_id,
   AddChange(change);
 }
 
+void TestChangeTracker::OnEventObserved(mojom::EventPtr event) {
+  Change change;
+  change.type = CHANGE_TYPE_EVENT_OBSERVED;
+  change.event_action = static_cast<int32_t>(event->action);
+  AddChange(change);
+}
+
 void TestChangeTracker::OnWindowSharedPropertyChanged(Id window_id,
                                                       String name,
                                                       Array<uint8_t> data) {