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..8323afc11a409c9d009c48bb7baa3d66f1494ed1 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",
@@ -22,15 +24,35 @@ component("ash_public_cpp") {
   defines = [ "ASH_PUBLIC_IMPLEMENTATION" ]
 
   deps = [
-    "//base",
+    "//mojo/common:common_custom_types",
+    "//skia/public/interfaces",
     "//ui/aura",
     "//ui/views/mus",
   ]
 
   public_deps = [
     "//ash/public/interfaces:interfaces_internal",
+    "//base",
+    "//ui/gfx",
   ]
 
   allow_circular_includes_from =
       [ "//ash/public/interfaces:interfaces_internal" ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "shelf_struct_traits_unittest.cc",
+  ]
+
+  deps = [
+    ":ash_public_cpp",
+
+    # TODO(crbug.com/702397): Rely on :ash_public_cpp for this instead.
+    "//ash/public/interfaces:interfaces_internal",
+    "//base",
+    "//testing/gtest",
+    "//ui/gfx:test_support",
+  ]
+}