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

mojo/public/cpp/bindings/lib/bindings_internal.h

 // Copyright 2013 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.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_LIB_BINDINGS_INTERNAL_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_BINDINGS_INTERNAL_H_
 
 #include <stdint.h>
 
 #include "mojo/public/cpp/bindings/lib/interface_id.h"
 #include "mojo/public/cpp/bindings/lib/template_util.h"
 #include "mojo/public/cpp/system/core.h"
 
+namespace WTF {
+class String;
+}
 namespace mojo {
 class String;
 
 template <typename T>
 class StructPtr;
 
 template <typename T>
 class InlinedStructPtr;
 
 namespace internal {
@@ skipping 131 matching lines @@
 template <typename S>
 struct WrapperTraits<S, true> {
   typedef typename S::Data_* DataType;
 };
 
 template <>
 struct WrapperTraits<String, false> {
   typedef String_Data* DataType;
 };
 
+template <>
+struct WrapperTraits<WTF::String, false> {
+  typedef String_Data* DataType;
+};
+
 }  // namespace internal
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_BINDINGS_INTERNAL_H_