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

mojo/public/cpp/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("../../../mojo_application.gni")
 
 source_set("tests") {
   testonly = true
 
   sources = [
@@ skipping 30 matching lines @@
     "stl_converters_unittest.cc",
     "string_unittest.cc",
     "struct_traits_unittest.cc",
     "struct_unittest.cc",
     "struct_with_traits_impl.cc",
     "struct_with_traits_impl.h",
     "sync_method_unittest.cc",
     "type_conversion_unittest.cc",
     "union_unittest.cc",
     "validation_unittest.cc",
+    "wtf_types_unittest.cc",
   ]
 
   deps = [
     ":mojo_public_bindings_test_utils",
     "//mojo/environment:chromium",
     "//mojo/message_pump",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/bindings:callback",
     "//mojo/public/cpp/system",
     "//mojo/public/cpp/test_support:test_utils",
     "//mojo/public/cpp/utility",
     "//mojo/public/interfaces/bindings/tests:test_associated_interfaces",
     "//mojo/public/interfaces/bindings/tests:test_interfaces",
     "//mojo/public/interfaces/bindings/tests:test_interfaces_blink",
     "//mojo/public/interfaces/bindings/tests:test_interfaces_chromium",
     "//mojo/public/interfaces/bindings/tests:test_interfaces_experimental",
     "//mojo/public/interfaces/bindings/tests:test_struct_traits_interfaces",
+    "//mojo/public/interfaces/bindings/tests:test_wtf_types",
+    "//mojo/public/interfaces/bindings/tests:test_wtf_types_wtf",
     "//testing/gtest",
+    "//third_party/WebKit/Source/wtf:test_support",
   ]
 }
 
 source_set("perftests") {
   testonly = true
 
   sources = [
     "bindings_perftest.cc",
   ]
 
@@ skipping 12 matching lines @@
 source_set("mojo_public_bindings_test_utils") {
   sources = [
     "validation_test_input_parser.cc",
     "validation_test_input_parser.h",
   ]
 
   deps = [
     "//mojo/public/c/system",
   ]
 }