mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/window_server.h

Index: components/mus/ws/window_server.h
diff --git a/components/mus/ws/window_server.h b/components/mus/ws/window_server.h
index 21b464e634d19350abb0993cd3fc50e4d3855412..23d00c6dac52352ecb9f0ce12c9313e1af30c98e 100644
--- a/components/mus/ws/window_server.h
+++ b/components/mus/ws/window_server.h
@@ -188,6 +188,12 @@ class WindowServer : public ServerWindowDelegate,
   void ProcessWillChangeWindowPredefinedCursor(ServerWindow* window,
                                                int32_t cursor_id);
 
+  // Sends an |event| to all WindowTrees belonging to |user_id| that might be
+  // observing events. Skips |ignore_tree| if it is non-null.
+  void SendToEventObservers(const ui::Event& event,
+                            const UserId& user_id,
+                            WindowTree* ignore_tree);
+
   // Sets a callback to be called whenever a ServerWindow is scheduled for
   // a [re]paint. This should only be called in a test configuration.
   void SetPaintCallback(const base::Callback<void(ServerWindow*)>& callback);