Mojo C++ bindings: switch the existing usage of StructTraits to use the new data view interface.

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) {
-    if (!input.is_null()) {
-      String result(input.storage(), input.size());
-      result.Swap(output);
-    } else {
-      *output = nullptr;
-    }
+    String result(input.storage(), input.size());
+    result.Swap(output);
     return true;
   }
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_STANDARD_H_