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

ash/public/cpp/BUILD.gn

Index: ash/public/cpp/BUILD.gn
diff --git a/ash/public/cpp/BUILD.gn b/ash/public/cpp/BUILD.gn
index 7b9c8b056581d50595e8241ce1240a6320c2f3d4..e78c322b254886853768990d36a92aaba42a46e8 100644
--- a/ash/public/cpp/BUILD.gn
+++ b/ash/public/cpp/BUILD.gn
@@ -9,6 +9,8 @@ component("ash_public_cpp") {
     "mus_property_mirror_ash.cc",
     "mus_property_mirror_ash.h",
     "session_types.h",
+    "shelf_item.cc",
+    "shelf_item.h",
     "shelf_types.cc",
     "shelf_types.h",
     "shell_window_ids.cc",
@@ -24,6 +26,7 @@ component("ash_public_cpp") {
   deps = [
     "//base",
     "//ui/aura",
+    "//ui/gfx",

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

Should this be public_deps, since if a client wants to use ShelfItem they must
include gfx for skia?

[msw, 2017/03/16 23:15:49]

Hmm, I guess so, and ditto for base for string16?. I wonder if it ever makes
sense to instead require that clients include the same dependency when it's only
needed by a small part of a component target. (that would alleviate users of
window_properties from needing to link against //ui/gfx, right?) I guess I could
make a separate shelf target here if that really became a burden.

     "//ui/views/mus",
   ]