views/mus: Add a test to verify that Widget receives events correctly.

components/mus/public/cpp/tests/window_tree_client_impl_private.h

Index: components/mus/public/cpp/tests/window_tree_client_impl_private.h
diff --git a/components/mus/public/cpp/tests/window_tree_client_impl_private.h b/components/mus/public/cpp/tests/window_tree_client_impl_private.h
index c1544aee444eca2c6df6d534fd2e1609ce133cbe..e1f745a06caaf9fe225d52e746a0d132e874520d 100644
--- a/components/mus/public/cpp/tests/window_tree_client_impl_private.h
+++ b/components/mus/public/cpp/tests/window_tree_client_impl_private.h
@@ -9,17 +9,23 @@
 
 #include "base/macros.h"
 
+namespace ui {
+class Event;
+}
+
 namespace mus {
 namespace mojom {
 class WindowTree;
 }
 
+class Window;
 class WindowTreeClientImpl;
 
 // Use to access implementation details of WindowTreeClientImpl.
 class WindowTreeClientImplPrivate {
  public:
   explicit WindowTreeClientImplPrivate(WindowTreeClientImpl* tree_client_impl);
+  explicit WindowTreeClientImplPrivate(Window* window);
   ~WindowTreeClientImplPrivate();
 
   uint32_t event_observer_id();
@@ -27,6 +33,9 @@ class WindowTreeClientImplPrivate {
   // Calls OnEmbed() on the WindowTreeClientImpl.
   void OnEmbed(mojom::WindowTree* window_tree);
 
+  // Pretends that |event| has been received from the window server.
+  void CallOnWindowInputEvent(Window* window, const ui::Event& event);
+
  private:
   WindowTreeClientImpl* tree_client_impl_;