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

components/mus/public/cpp/window.h

Index: components/mus/public/cpp/window.h
diff --git a/components/mus/public/cpp/window.h b/components/mus/public/cpp/window.h
index 9ce601f291c97c90d3924535034e37c1a6e75d78..883dec239621f3b11a989701b5707251ac6b1db5 100644
--- a/components/mus/public/cpp/window.h
+++ b/components/mus/public/cpp/window.h
@@ -32,6 +32,7 @@ class WindowObserver;
 class WindowSurface;
 class WindowSurfaceBinding;
 class WindowTreeClientImpl;
+class WindowTreeClientImplPrivate;
 class WindowTreeConnection;
 
 namespace {
@@ -229,6 +230,7 @@ class Window {
  private:
   friend class WindowPrivate;
   friend class WindowTreeClientImpl;
+  friend class WindowTreeClientImplPrivate;
 
   Window(WindowTreeConnection* connection, Id id);