mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/event_dispatcher_delegate.h

Index: components/mus/ws/event_dispatcher_delegate.h
diff --git a/components/mus/ws/event_dispatcher_delegate.h b/components/mus/ws/event_dispatcher_delegate.h
index a762df51a8ff2c25e20a7d1ae72f21b0eb390f6d..c377d1be4bc1a56a6c77f12c7b48b5cceda390a7 100644
--- a/components/mus/ws/event_dispatcher_delegate.h
+++ b/components/mus/ws/event_dispatcher_delegate.h
@@ -41,6 +41,10 @@ class EventDispatcherDelegate {
                                           const ui::Event& event,
                                           Accelerator* accelerator) = 0;
 
+  // Called when event dispatch could not find a target. OnAccelerator may still
+  // be called.
+  virtual void OnEventTargetNotFound(const ui::Event& event) = 0;
+
  protected:
   virtual ~EventDispatcherDelegate() {}
 };