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

ash/public/cpp/shelf_types.h

Index: ash/public/cpp/shelf_types.h
diff --git a/ash/public/cpp/shelf_types.h b/ash/public/cpp/shelf_types.h
index 5cec6237288886b1e1f26d4f02ce68489d114667..3fad7e2cc6772331898ecd9e43a84359f12ec7c6 100644
--- a/ash/public/cpp/shelf_types.h
+++ b/ash/public/cpp/shelf_types.h
@@ -89,8 +89,8 @@ enum ShelfAction {
   SHELF_ACTION_APP_LIST_SHOWN,
 };
 
-typedef int ShelfID;
-const int kInvalidShelfID = 0;
+typedef int64_t ShelfID;

[James Cook, 2017/03/16 16:48:50]

nit: using ShelfID = int64_t

Also, are there any useful docs you can add for shelf id, like are they
sequential, are they persistent across restarts of chrome, can they be safely
written to disk, etc.?

Also also, is there a particular motivation to make these int64? Just so they
don't get confused with regular int? If you want to prevent int/shelfid
confusion I wonder if making the ID type unsigned would help generate compiler
warnings. I can't remember if we disable those everywhere anymore.

[msw, 2017/03/16 23:15:50]

Done.
 
Added a comment with some answers.
My motivation was to pick a concrete type that would match mojo, which doesn't
handle 32-/64-bit int type size inference from the platform. They are only
non-negative and generally small (# of shelf items), so I changed this to
uint32_t ... I might need to pair them with a client id (similar to mus windows)
later on.

+const int64_t kInvalidShelfID = 0;
 
 // The type of a shelf item.
 enum ShelfItemType {