Revert of mus: Enable system modal windows

components/mus/ws/server_window.h

Index: components/mus/ws/server_window.h
diff --git a/components/mus/ws/server_window.h b/components/mus/ws/server_window.h
index 2d7070a39410d677e55246a8764a3383ada81c07..a518ff085e40a8893142a183e13539645a4e86ee 100644
--- a/components/mus/ws/server_window.h
+++ b/components/mus/ws/server_window.h
@@ -100,8 +100,7 @@
   ServerWindow* GetChildWindow(const WindowId& id);
 
   // Transient window management.
-  // Adding transient child fails if the child window is modal to system.
-  bool AddTransientWindow(ServerWindow* child);
+  void AddTransientWindow(ServerWindow* child);
   void RemoveTransientWindow(ServerWindow* child);
 
   ServerWindow* transient_parent() { return transient_parent_; }
@@ -111,6 +110,17 @@
 
   bool is_modal() const { return is_modal_; }
   void SetModal();
+
+  bool IsBlockedByModalWindow() const;
+
+  // Returns the window that events targeted to this window should be retargeted
+  // to; according to modal windows. If any modal window is blocking this
+  // window, returns the topmost one; otherwise, returns this window.
+  const ServerWindow* GetModalTarget() const;
+  ServerWindow* GetModalTarget() {
+    return const_cast<ServerWindow*>(
+        static_cast<const ServerWindow*>(this)->GetModalTarget());
+  }
 
   // Returns true if this contains |window| or is |window|.
   bool Contains(const ServerWindow* window) const;