Mojo C++ bindings: support mapping mojo string to WTF::String.

mojo/public/interfaces/bindings/tests/test_wtf_types.mojom

Index: mojo/public/interfaces/bindings/tests/test_wtf_types.mojom
diff --git a/mojo/public/interfaces/bindings/tests/test_wtf_types.mojom b/mojo/public/interfaces/bindings/tests/test_wtf_types.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..f7763093a52fa5548aa500f364171f7947c3f72c
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+++ b/mojo/public/interfaces/bindings/tests/test_wtf_types.mojom
@@ -0,0 +1,14 @@
+// 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.test;
+
+struct TestWTFCodeGeneration {
+  string str;
+  string? nullable_str;
+};
+
+interface TestWTF {
+  EchoString(string? str) => (string? str);
+};