Add simple mash context menu support.

mash/shelf/public/interfaces/context_menu.mojom

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module mash.shelf.mojom;
+
+struct ContextMenuItem {

[sky, 2016/03/18 20:30:57]

I have a feeling we're going to need this structure in other places. How about
moving to mash/common/public/interfaces/context_menu_item.mojom?

[sky, 2016/03/18 20:31:34]

Ignore me on this one. While I think we'll need this for other code, we can move
it when that happens.

+  enum Type {
+    ITEM,
+    CHECK,
+    RADIO,
+    SEPARATOR,
+    SUBMENU,
+  };
+
+  Type type;
+  uint32 command_id;
+  string? label;
+  array<ContextMenuItem>? submenu;
+  bool enabled;
+  bool checked;
+  uint32 radio_group_id;
+};
+
+interface ContextMenu {
+  // TODO(msw) Generalize a profile prefs provider/observer, etc.
+  GetItemsAndInfo(uint32 shelf_item_type, int32 shelf_item_id) =>
+      (array<ContextMenuItem> items,
+       bool can_user_modify_shelf_auto_hide_behavior,
+       bool is_guest_session);
+
+  ExecuteCommand(int32 command_id, int32 event_flags);
+};