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

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

 // 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.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_LIB_STRING_SERIALIZATION_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_STRING_SERIALIZATION_H_
 
 #include <stddef.h>
 #include <string.h>
 
@@ skipping 40 matching lines @@
              CallWithContext(Traits::GetSize, input, custom_context));
     }
     *output = result;
 
     CustomContextHelper<Traits>::TearDown(input, custom_context);
   }
 
   static bool Deserialize(String_Data* input,
                           UserType* output,
                           SerializationContext* context) {
+    if (!input)
+      return CallSetToNullIfExists<Traits>(output);
     return Traits::Read(StringDataView(input), output);
   }
 };
 
 }  // namespace internal
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_STRING_SERIALIZATION_H_