Mojo C++ bindigns: fix generated namespace when variant is specified.

mojo/public/interfaces/bindings/tests/BUILD.gn

 # Copyright 2014 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("../../../tools/bindings/mojom.gni")
 
 mojom("test_interfaces") {
   testonly = true
   sources = [
     "math_calculator.mojom",
@@ skipping 98 matching lines @@
 mojom("test_wtf_types_blink") {
   testonly = true
 
   sources = [
     "test_wtf_types.mojom",
   ]
 
   for_blink = true
   variant = "blink"
 }
+
+mojom("test_variant") {
+  testonly = true
+
+  sources = [
+    "test_variant.mojom",
+  ]
+
+  variant = "variant_test"
+}