mus: Add EventObserver to allow passively listening to UI events

components/mus/public/cpp/lib/window_tree_client_impl.cc

Index: components/mus/public/cpp/lib/window_tree_client_impl.cc
diff --git a/components/mus/public/cpp/lib/window_tree_client_impl.cc b/components/mus/public/cpp/lib/window_tree_client_impl.cc
index 6e26c35de28232d7c6649369a7cf43d8a9e9fe20..c377a8ffc75064da17b0fb218d69447c3c597d40 100644
--- a/components/mus/public/cpp/lib/window_tree_client_impl.cc
+++ b/components/mus/public/cpp/lib/window_tree_client_impl.cc
@@ -865,7 +865,11 @@ void WindowTreeClientImpl::OnWindowSharedPropertyChanged(
 
 void WindowTreeClientImpl::OnWindowInputEvent(uint32_t event_id,
                                               Id window_id,
-                                              mojom::EventPtr event) {
+                                              mojom::EventPtr event,
+                                              uint32_t event_observer_id) {
+  // TODO(jamescook): If event_observer_id is non-zero, also route the event to
+  // PointerWatchers.
+
   Window* window = GetWindowByServerId(window_id);
   if (!window || !window->input_event_handler_) {
     tree_->OnWindowInputEventAck(event_id, mojom::EventResult::UNHANDLED);
@@ -885,6 +889,11 @@ void WindowTreeClientImpl::OnWindowInputEvent(uint32_t event_id,
     ack_callback->Run(mojom::EventResult::UNHANDLED);
 }
 
+void WindowTreeClientImpl::OnEventObserved(mojom::EventPtr event_in,
+                                           uint32_t event_observer_id) {
+  // TODO(jamescook): Route the observed event to PointerWatchers.
+}
+
 void WindowTreeClientImpl::OnWindowFocused(Id focused_window_id) {
   Window* focused_window = GetWindowByServerId(focused_window_id);
   InFlightFocusChange new_change(this, focused_window);