Multi-select <select> in 'external popup window'

content/browser/renderer_host/popup_menu_helper_mac.mm

Index: content/browser/renderer_host/popup_menu_helper_mac.mm
diff --git a/content/browser/renderer_host/popup_menu_helper_mac.mm b/content/browser/renderer_host/popup_menu_helper_mac.mm
index 9f371486693f8d20bbc04c008cccf67c0c0a84b8..5befbe10539aed6aa312c7514ba5b6a4482e85cf 100644
--- a/content/browser/renderer_host/popup_menu_helper_mac.mm
+++ b/content/browser/renderer_host/popup_menu_helper_mac.mm
@@ -33,7 +33,11 @@ void PopupMenuHelper::ShowPopupMenu(
     double item_font_size,
     int selected_item,
     const std::vector<WebMenuItem>& items,
-    bool right_aligned) {
+    bool right_aligned,
+    bool allow_multiple_selection) {
+  // Only single selection list boxes show a popup on Mac.
+  DCHECK(!allow_multiple_selection);
+
   // Retain the Cocoa view for the duration of the pop-up so that it can't be
   // dealloced if my Destroy() method is called while the pop-up's up (which
   // would in turn delete me, causing a crash once the -runMenuInView