mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/window_tree.h

Index: components/mus/ws/window_tree.h
diff --git a/components/mus/ws/window_tree.h b/components/mus/ws/window_tree.h
index ab8e62430d50ff6c05aee7172d83ae7ed5f9990c..40d44f771050d6b18495d5a731f57540ac05b8c7 100644
--- a/components/mus/ws/window_tree.h
+++ b/components/mus/ws/window_tree.h
@@ -39,6 +39,7 @@ namespace ws {
 class AccessPolicy;
 class DisplayManager;
 class Display;
+class EventTester;
 class ServerWindow;
 class TargetedEvent;
 class WindowManagerState;
@@ -207,6 +208,9 @@ class WindowTree : public mojom::WindowTree,
                                      const ServerWindow* transient_window,
                                      bool originated_change);
 
+  // Sends this event to the client if it matches an active event observer.
+  void SendToEventObserver(const ui::Event& event);
+
  private:
   friend class test::WindowTreeTestApi;
 
@@ -335,6 +339,8 @@ class WindowTree : public mojom::WindowTree,
       override;
   void SetCapture(uint32_t change_id, Id window_id) override;
   void ReleaseCapture(uint32_t change_id, Id window_id) override;
+  void SetEventObserver(mojom::EventMatcherPtr matcher) override;
+  void ClearEventObserver() override;
   void SetWindowBounds(uint32_t change_id,
                        Id window_id,
                        mojo::RectPtr bounds) override;
@@ -429,6 +435,10 @@ class WindowTree : public mojom::WindowTree,
 
   uint32_t event_ack_id_;
 
+  // Set when the client is using SetEventObserver() to observe events,
+  // otherwise null.
+  std::unique_ptr<EventTester> event_observer_tester_;
+
   // WindowManager the current event came from.
   WindowManagerState* event_source_wms_ = nullptr;