mash: Port ash_sysui ShelfDelegateMus impl to mojo:ash.

mash/shelf/public/interfaces/BUILD.gn

Index: mash/shelf/public/interfaces/BUILD.gn
diff --git a/mash/shelf/public/interfaces/BUILD.gn b/mash/shelf/public/interfaces/BUILD.gn
deleted file mode 100644
index 65e038ed517bc85281f4e7160880425792a45a3b..0000000000000000000000000000000000000000
--- a/mash/shelf/public/interfaces/BUILD.gn
+++ /dev/null
@@ -1,18 +0,0 @@
-# Copyright 2016 The Chromium Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
-
-import("//mojo/public/tools/bindings/mojom.gni")
-
-mojom("interfaces") {
-  sources = [
-    "shelf.mojom",
-    "shelf_constants.mojom",
-  ]
-
-  public_deps = [
-    "//skia/public/interfaces",
-  ]
-
-  use_new_wrapper_types = false
-}