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

mojo/public/cpp/bindings/string_traits_wtf.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_WTF_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_WTF_H_
 
 #include "mojo/public/cpp/bindings/lib/bindings_internal.h"
 #include "mojo/public/cpp/bindings/string_traits.h"
 #include "third_party/WebKit/Source/wtf/text/WTFString.h"
 
 namespace mojo {
 
 template <>
 struct StringTraits<WTF::String> {
   static bool IsNull(const WTF::String& input) { return input.isNull(); }
+  static void SetToNull(WTF::String* output);
 
   static void* SetUpContext(const WTF::String& input);
   static void TearDownContext(const WTF::String& input, void* context);
 
   static size_t GetSize(const WTF::String& input, void* context);
 
   static const char* GetData(const WTF::String& input, void* context);
 
   static bool Read(StringDataView input, WTF::String* output);
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_STRING_TRAITS_WTF_H_