Mojo C++ bindings: add detailed comments for String/Array/StructTraits.

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