mus: Add PointerWatcher for passively observing mouse and touch events

mash/wm/root_window_controller.cc

Index: mash/wm/root_window_controller.cc
diff --git a/mash/wm/root_window_controller.cc b/mash/wm/root_window_controller.cc
index 195f8368ed32865b744b104c70dd605d98495232..5a07a2684e60ac29220db7215d31c3c5be14e5e3 100644
--- a/mash/wm/root_window_controller.cc
+++ b/mash/wm/root_window_controller.cc
@@ -157,6 +157,10 @@ void RootWindowController::OnConnectionLost(
   delete this;
 }
 
+void RootWindowController::OnEventObserved(const ui::Event& event) {
+  // Does not use EventObservers.
+}
+
 void RootWindowController::OnWindowDestroyed(mus::Window* window) {
   DCHECK_EQ(window, root_);
   app_->OnRootWindowDestroyed(this);