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

ash/shelf/shelf_view_unittest.cc

Index: ash/shelf/shelf_view_unittest.cc
diff --git a/ash/shelf/shelf_view_unittest.cc b/ash/shelf/shelf_view_unittest.cc
index f6722e837926c73d1ceac970ee7081031fa39285..f6bdd296415fc534e973d5c832d401eeaf637a49 100644
--- a/ash/shelf/shelf_view_unittest.cc
+++ b/ash/shelf/shelf_view_unittest.cc
@@ -257,7 +257,7 @@ class TestShelfDelegateForShelfView : public TestShelfDelegate {
   // ShelfDelegate overrides:
   ShelfID GetShelfIDForAppID(const std::string& app_id) override {
     ShelfID id = 0;
-    EXPECT_TRUE(base::StringToInt(app_id, &id));
+    EXPECT_TRUE(base::StringToInt64(app_id, &id));
     return id;
   }
 
@@ -553,7 +553,7 @@ class ShelfViewTest : public AshTestBase {
       int from,
       int to,
       ShelfView* shelf_view,
-      const std::vector<std::pair<int, views::View*>>& expected_id_map) {
+      const std::vector<std::pair<ShelfID, views::View*>>& expected_id_map) {
     views::View* dragged_button =
         SimulateDrag(ShelfView::MOUSE, from, to, true);
     shelf_view->PointerReleasedOnButton(dragged_button, ShelfView::MOUSE,