Mojo C++ bindings: expose public <struct>DataView and StringDataView.

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 size_t GetSize(const String& input) { return input.size(); }
 
   static const char* GetData(const String& input) { return input.data(); }
 
-  // TODO(yzshen): Use a public type, such as mojo::String::DataView, for
-  // |input|.
-  static bool Read(internal::String_Data* input, String* output) {
-    if (input) {
-      String result(input->storage(), input->size());
+  static bool Read(StringDataView input, String* output) {
+    if (!input.is_null()) {
+      String result(input.storage(), input.size());
       result.Swap(output);
     } else {
       *output = nullptr;
     }
     return true;
   }
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_STANDARD_H_