Remove mojo::String.

mojo/public/cpp/bindings/string_traits_standard.h

-// 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.
-
-#ifndef MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_STANDARD_H_
-#define MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_STANDARD_H_
-
-#include "mojo/public/cpp/bindings/string.h"
-#include "mojo/public/cpp/bindings/string_traits.h"
-
-namespace mojo {
-
-template <>
-struct StringTraits<String> {
-  static bool IsNull(const String& input) { return input.is_null(); }
-  static void SetToNull(String* output) { *output = nullptr; }
-
-  static size_t GetSize(const String& input) { return input.size(); }
-
-  static const char* GetData(const String& input) { return input.data(); }
-
-  static bool Read(StringDataView input, String* output) {
-    String result(input.storage(), input.size());
-    result.Swap(output);
-    return true;
-  }
-};
-
-}  // namespace mojo
-
-#endif  // MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_STANDARD_H_