mus: Add PointerWatcher for passively observing mouse and touch events

mash/wm/window_manager_unittest.cc

Index: mash/wm/window_manager_unittest.cc
diff --git a/mash/wm/window_manager_unittest.cc b/mash/wm/window_manager_unittest.cc
index 24720dcd051f46c6faeaeadfdaffbae01db88f32..932e49d2e23f11690173561972f114ffc7d08bfa 100644
--- a/mash/wm/window_manager_unittest.cc
+++ b/mash/wm/window_manager_unittest.cc
@@ -29,6 +29,7 @@ class WindowTreeDelegateImpl : public mus::WindowTreeDelegate {
   // mus::WindowTreeDelegate:
   void OnEmbed(mus::Window* root) override {}
   void OnConnectionLost(mus::WindowTreeConnection* connection) override {}
+  void OnEventObserved(const ui::Event& event) override {}
 
   DISALLOW_COPY_AND_ASSIGN(WindowTreeDelegateImpl);
 };