Fix -Wimplicit-fallthrough warnings for Chrome OS.

Adds a missing break; without this change as shelf context menu buttons
would be treated as submenus in release builds. Since the button
item type is not yet used, this is not observable in practice.

This CL was uploaded by git cl split.

R=msw@chromium.org

Bug: 177475
Change-Id: I913b81eef071c6a0dcc4f67c206d551cdf5be639
Reviewed-on: https://chromium-review.googlesource.com/899787
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Stefan Kuhne <skuhne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534435}

ash/shelf/shelf_context_menu_model.cc

@@ -188,6 +188,7 @@ void ShelfContextMenuModel::AddItems(ui::SimpleMenuModel* model,
         break;
       case ui::MenuModel::TYPE_BUTTON_ITEM:
         NOTREACHED() << "TYPE_BUTTON_ITEM is not yet supported.";
+        break;
       case ui::MenuModel::TYPE_SUBMENU:
         if (item->submenu.has_value()) {
           std::unique_ptr<ui::MenuModel> submenu =