Add Mojo constant for "ash" and replace hardcoded strings

mash/simple_wm/public/interfaces/BUILD.gn

Index: mash/simple_wm/public/interfaces/BUILD.gn
diff --git a/mash/simple_wm/public/interfaces/BUILD.gn b/mash/simple_wm/public/interfaces/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..31de69f8933112ebc1582b5d0319e0c9e0e89fcc
--- /dev/null
+++ b/mash/simple_wm/public/interfaces/BUILD.gn
@@ -0,0 +1,17 @@
+# Copyright 2017 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")
+
+group("interfaces") {
+  public_deps = [
+    ":constants",
+  ]
+}
+
+mojom("constants") {
+  sources = [
+    "constants.mojom",
+  ]
+}