Datagrid: Make the header context menu keyboard accessible

This change adds a sub menu item to the datagrid context menu that
allows the Header context menu to be accessible from the
contextMenuKey press in the body of the datagrid. This is similar
to the "Sort By" menu option added previously for the same
purpose.

Currently, the header options are used in the network tool and the
protocol monitor.

Before: https://imgur.com/oGh9fwr

After: https://imgur.com/G37Cts3

Bug: 963183
Change-Id: I4d02bed0dc9924be7fd415cad4053711a3aeb044
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1891662
Commit-Queue: Brandon Goddard <[email protected]>
Reviewed-by: Lorne Mitchell <[email protected]>
Reviewed-by: Jack Lynch <[email protected]>
diff --git a/front_end/ui/ContextMenu.js b/front_end/ui/ContextMenu.js
index a3a76c9..6eb13df 100644
--- a/front_end/ui/ContextMenu.js
+++ b/front_end/ui/ContextMenu.js
@@ -207,7 +207,7 @@
 /**
  * @unrestricted
  */
-class SubMenu extends Item {
+export class SubMenu extends Item {
   /**
    * @param {?ContextMenu} contextMenu
    * @param {string=} label
@@ -589,6 +589,9 @@
  */
 UI.ContextMenuSection = Section;
 
+/** @constructor */
+UI.ContextSubMenu = SubMenu;
+
 /**
  * @interface
  */