Bug 1820203 - Don't consider dialogs with BorderStyle::{Default,All} as undecorated. r=stransky

Bug 1794577 made this visible but the bug was really introduced in
bug 1816436.

Differential Revision: https://phabricator.services.mozilla.com/D173428

widget/gtk/nsWindow.cpp

@@ -8760,6 +8760,8 @@ bool nsWindow::IsAlwaysUndecoratedWindow() const {
     return true;
   }
   if (mWindowType == WindowType::Dialog &&
+      mBorderStyle != BorderStyle::Default &&
+      mBorderStyle != BorderStyle::All &&
       !(mBorderStyle & BorderStyle::Title) &&
       !(mBorderStyle & BorderStyle::ResizeH)) {
     return true;