Port messages sent by WebIDBFactoryImpl to Mojo.

mojo/public/cpp/bindings/array_traits_stl.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_STL_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_ARRAY_TRAITS_STL_H_
 
+#include <map>
+#include <set>
 #include <vector>
 
 #include "mojo/public/cpp/bindings/array_traits.h"
 
 namespace mojo {
 
 template <typename T>
 struct ArrayTraits<std::vector<T>> {
   using Element = T;
 
@@ skipping 60 matching lines @@
     return input.begin();
   }
   static void AdvanceIterator(ConstIterator& iterator) {
     ++iterator;
   }
   static const T& GetValue(ConstIterator& iterator) {
     return *iterator;
   }
 };
 
+// This ArrayTraits specialization is used only for serialization.
+template <typename K, typename V>
+struct ArrayTraits<std::map<K, V>> {

[Ken Rockot(use gerrit already), 2016/10/14 21:50:47]

I think this is probably a bit confusing to do. I would create a cheap
intermediate type like mojo::MapValueArrayView<T>, and then create the
ArrayTraits over that. Then your StructTraits have to explicitly return a
MapValueArrayView. Otherwise it's easy to accidentally flatten a map into an
array.

[Reilly Grant (use Gerrit), 2016/10/15 00:13:42]

Done. Let me know if there's a way to make usage of this wrapper even less
verbose.

+  using Element = V;
+  using ConstIterator = typename std::map<K, V>::const_iterator;
+
+  static bool IsNull(const std::map<K, V>& input) {
+    // std::map<> is always converted to non-null mojom array.
+    return false;
+  }
+
+  static size_t GetSize(const std::map<K, V>& input) { return input.size(); }
+
+  static ConstIterator GetBegin(const std::map<K, V>& input) {
+    return input.begin();
+  }
+  static void AdvanceIterator(ConstIterator& iterator) { ++iterator; }
+  static const V& GetValue(ConstIterator& iterator) { return iterator->second; }
+};
+
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_ARRAY_TRAITS_STL_H_