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

mojo/public/cpp/bindings/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.
 
 source_set("bindings") {
   sources = [
     "array.h",
     "associated_binding.h",
     "associated_group.h",
     "associated_interface_ptr.h",
@@ skipping 63 matching lines @@
     "lib/string_serialization.h",
     "lib/sync_handle_watcher.cc",
     "lib/sync_handle_watcher.h",
     "lib/union_accessor.h",
     "lib/validate_params.h",
     "lib/validation_errors.cc",
     "lib/validation_errors.h",
     "lib/validation_util.cc",
     "lib/validation_util.h",
     "lib/value_traits.h",
+
+    # Include the .h but not the .cc file. The .h file is used by
+    # serialization_forward.h.
+    "lib/wtf_string_serialization.h",
     "map.h",
     "message.h",
     "message_filter.h",
     "no_interface.h",
     "stl_converters.h",
     "string.h",
     "strong_binding.h",
     "struct_ptr.h",
     "struct_traits.h",
     "type_converter.h",
@@ skipping 19 matching lines @@
     "lib/shared_data.h",
     "lib/shared_ptr.h",
     "lib/template_util.h",
   ]
 
   deps = [
     "//base",
     "//mojo/public/cpp/system",
   ]
 }
+
+source_set("wtf_support") {
+  sources = [
+    "lib/wtf_serialization.h",
+    "lib/wtf_string_serialization.cc",
+    "lib/wtf_string_serialization.h",
+  ]
+
+  public_deps = [
+    ":bindings",
+    "//third_party/WebKit/Source/wtf",
+  ]
+
+  public_configs = [ "//third_party/WebKit/Source:config" ]
+}