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

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 96 matching lines @@
 
   variant = "blink"
   typemaps = [ "blink_test.typemap" ]
 
   with_environment = false
 
   public_deps = [
     ":test_interfaces",
   ]
 }
+
+mojom("test_wtf_types") {
+  testonly = true
+
+  sources = [
+    "test_wtf_types.mojom",
+  ]
+
+  with_environment = false
+}
+
+mojom("test_wtf_types_wtf") {
+  testonly = true
+
+  sources = [
+    "test_wtf_types.mojom",
+  ]
+
+  deps = [
+    ":test_wtf_types",
+  ]
+
+  use_wtf_types = true
+  variant = "wtf"
+
+  with_environment = false
+}