mash: Fix ShelfItem mojo struct; add enums and traits.

ash/common/shelf/shelf_window_watcher.cc

 // Copyright 2013 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.
 
 #include "ash/common/shelf/shelf_window_watcher.h"
 
 #include <memory>
 #include <utility>
 
 #include "ash/common/shelf/shelf_constants.h"
@@ skipping 154 matching lines @@
   window->SetProperty(kShelfIDKey, id);
   model_->SetShelfItemDelegate(id,
                                base::MakeUnique<ShelfWindowWatcherItemDelegate>(
                                    id, WmWindow::Get(window)));
   // Panels are inserted on the left so as not to push all existing panels over.
   model_->AddAt(item.type == TYPE_APP_PANEL ? 0 : model_->item_count(), item);
 }
 
 void ShelfWindowWatcher::RemoveShelfItem(aura::Window* window) {
   user_windows_with_items_.erase(window);
-  int shelf_id = window->GetProperty(kShelfIDKey);
+  ShelfID shelf_id = window->GetProperty(kShelfIDKey);
   DCHECK_NE(shelf_id, kInvalidShelfID);
   int index = model_->ItemIndexByID(shelf_id);
   DCHECK_GE(index, 0);
   model_->RemoveItemAt(index);
   window->SetProperty(kShelfIDKey, kInvalidShelfID);
 }
 
 void ShelfWindowWatcher::OnContainerWindowDestroying(aura::Window* container) {
   observed_container_windows_.Remove(container);
 }
@@ skipping 74 matching lines @@
   }
 }
 
 void ShelfWindowWatcher::OnDisplayRemoved(const display::Display& old_display) {
 }
 
 void ShelfWindowWatcher::OnDisplayMetricsChanged(const display::Display&,
                                                  uint32_t) {}
 
 }  // namespace ash