Mojo: add native type mapping for base::FilePath

mojo/common/test_common_custom_types.mojom

Index: mojo/common/test_common_custom_types.mojom
diff --git a/mojo/common/test_common_custom_types.mojom b/mojo/common/test_common_custom_types.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..0d2f165a8099e084075ac3b08aecdac5d92480b9
--- /dev/null
+++ b/mojo/common/test_common_custom_types.mojom
@@ -0,0 +1,12 @@
+// 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 mojo.common.test;
+
+import "mojo/common/common_custom_types.mojom";
+
+interface TestFilePath {
+  BounceFilePath(mojo.common.mojom.FilePath in)
+      => (mojo.common.mojom.FilePath out);
+};