mash: Serialize ShelfIDs for property conversion and transport.

ash/mus/window_manager.cc

Index: ash/mus/window_manager.cc
diff --git a/ash/mus/window_manager.cc b/ash/mus/window_manager.cc
index ee4d4cf7d8652655f0d43dcf4fbbc8ee546ab3a4..98e41b45ab8e8e879920922e0bc3c53e56d7386e 100644
--- a/ash/mus/window_manager.cc
+++ b/ash/mus/window_manager.cc
@@ -90,8 +90,7 @@ WindowManager::WindowManager(service_manager::Connector* connector,
       ui::mojom::WindowManager::kRenderParentTitleArea_Property,
       aura::PropertyConverter::CreateAcceptAnyValueCallback());
   property_converter_->RegisterProperty(
-      kShelfIDKey, ui::mojom::WindowManager::kShelfID_Property,
-      aura::PropertyConverter::CreateAcceptAnyValueCallback());
+      kShelfIDKey, ui::mojom::WindowManager::kShelfID_Property);
   property_converter_->RegisterProperty(
       kShelfItemTypeKey, ui::mojom::WindowManager::kShelfItemType_Property,
       base::Bind(&IsValidShelfItemType));