Convert //mash to define service names in mojom

mash/webtest/public/interfaces/constants.mojom

Index: mash/webtest/public/interfaces/constants.mojom
diff --git a/mash/webtest/public/interfaces/constants.mojom b/mash/webtest/public/interfaces/constants.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..c7bf6519c6b54ef3005259d66c6c50545d36bea8
--- /dev/null
+++ b/mash/webtest/public/interfaces/constants.mojom
@@ -0,0 +1,7 @@
+// 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.
+
+module mash.webtest.mojom;
+
+const string kServiceName = "webtest";