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

mojo/public/cpp/bindings/lib/array_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_LIB_ARRAY_TRAITS_WTF_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_ARRAY_TRAITS_WTF_H_
 
 #include "mojo/public/cpp/bindings/array_traits.h"
 #include "mojo/public/cpp/bindings/wtf_array.h"
 
 namespace mojo {
 
 template <typename U>
 struct ArrayTraits<WTFArray<U>> {
   using Element = U;
 
   static bool IsNull(const WTFArray<U>& input) { return input.is_null(); }
+  static void SetToNull(WTFArray<U>* output) { *output = nullptr; }
 
   static size_t GetSize(const WTFArray<U>& input) { return input.size(); }
 
   static U* GetData(WTFArray<U>& input) { return &input.front(); }
 
   static const U* GetData(const WTFArray<U>& input) { return &input.front(); }
 
   static U& GetAt(WTFArray<U>& input, size_t index) { return input[index]; }
 
   static const U& GetAt(const WTFArray<U>& input, size_t index) {
     return input[index];
   }
 
   static void Resize(WTFArray<U>& input, size_t size) { input.resize(size); }
-
-  static void SetToNull(WTFArray<U>& input) { input = nullptr; }
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_ARRAY_TRAITS_WTF_H_