Reland: mus: Enable system modal windows

components/mus/ws/window_manager_state.h

Index: components/mus/ws/window_manager_state.h
diff --git a/components/mus/ws/window_manager_state.h b/components/mus/ws/window_manager_state.h
index 4af382a7048dbaabd52be2bf280e57fb20da163d..aca7036fa52576ef5856b3694b66bd64414a896f 100644
--- a/components/mus/ws/window_manager_state.h
+++ b/components/mus/ws/window_manager_state.h
@@ -75,14 +75,11 @@ class WindowManagerState : public EventDispatcherDelegate {
     return event_dispatcher_.capture_window();
   }
 
-  // Checks if |modal_window| is a visible modal window that blocks current
-  // capture window and if that's the case, releases the capture.
   void ReleaseCaptureBlockedByModalWindow(const ServerWindow* modal_window);
-
-  // Checks if the current capture window is blocked by any visible modal window
-  // and if that's the case, releases the capture.
   void ReleaseCaptureBlockedByAnyModalWindow();
 
+  void AddSystemModalWindow(ServerWindow* window);
+
   // Returns true if this is the WindowManager of the active user.
   bool IsActive() const;