MacViews: Fix combobox keyboard shortcuts.

ui/views/controls/combobox/combobox.h

Index: ui/views/controls/combobox/combobox.h
diff --git a/ui/views/controls/combobox/combobox.h b/ui/views/controls/combobox/combobox.h
index 3842a45f122eda0b7316d3bbab80de68529900c3..a7a7cae22c7d75bed42ea373efca83797e772461 100644
--- a/ui/views/controls/combobox/combobox.h
+++ b/ui/views/controls/combobox/combobox.h
@@ -36,11 +36,13 @@ class PrefixSelector;
 // Combobox has two distinct parts, the drop down arrow and the text. Combobox
 // offers two distinct behaviors:
 // * STYLE_NORMAL: typical combobox, clicking on the text and/or button shows
-// the drop down, arrow keys change selection, selected index can be changed by
-// the user to something other than the first item.
+//   the drop down, arrow keys change selection or show the menu depending on
+//   the platform, selected index can be changed by the user to something other
+//   than the first item.
 // * STYLE_ACTION: clicking on the text notifies the listener. The menu can be
-// shown only by clicking on the arrow. The selected index is always reverted to
-// 0 after the listener is notified.
+//   shown only by clicking on the arrow, except on Mac where it can be shown
+//   through the keyboard. The selected index is always reverted to 0 after the
+//   listener is notified.
 class VIEWS_EXPORT Combobox : public View,
                               public PrefixDelegate,
                               public ButtonListener {