Reland: mus: Enable system modal windows

components/mus/ws/test_utils.h

Index: components/mus/ws/test_utils.h
diff --git a/components/mus/ws/test_utils.h b/components/mus/ws/test_utils.h
index f764f950c37ddab2e8525a07da3fac705699c1ca..326de6c7f5b7ab7d563b2bc4c2cbd29b4449df79 100644
--- a/components/mus/ws/test_utils.h
+++ b/components/mus/ws/test_utils.h
@@ -111,10 +111,11 @@ class EventDispatcherTestApi {
 
   bool AreAnyPointersDown() const { return ed_->AreAnyPointersDown(); }
   bool is_mouse_button_down() const { return ed_->mouse_button_down_; }
-  bool IsObservingWindow(ServerWindow* window) {
-    return ed_->IsObservingWindow(window);
-  }
+  bool IsWindowPointerTarget(const ServerWindow* window) const;
   int NumberPointerTargetsForWindow(ServerWindow* window);
+  ServerWindow* GetActiveSystemModalWindow() const {
+    return ed_->GetActiveSystemModalWindow();
+  }
 
  private:
   EventDispatcher* ed_;