Allow Mojo structs as map keys

mojo/public/cpp/bindings/map_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_MAP_TRAITS_STL_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_MAP_TRAITS_STL_H_
 
 #include <map>
 #include <unordered_map>
 
@@ skipping 76 matching lines @@
 
   static void AdvanceIterator(ConstIterator& iterator) { iterator++; }
   static void AdvanceIterator(Iterator& iterator) { iterator++; }
 
   static const K& GetKey(Iterator& iterator) { return iterator->first; }
   static const K& GetKey(ConstIterator& iterator) { return iterator->first; }
 
   static V& GetValue(Iterator& iterator) { return iterator->second; }
   static const V& GetValue(ConstIterator& iterator) { return iterator->second; }
 
-  static bool Insert(std::unordered_map<K, V>& input, const K& key, V&& value) {
-    input.insert(std::make_pair(key, std::forward<V>(value)));
-    return true;
-  }
-  static bool Insert(std::unordered_map<K, V>& input,
-                     const K& key,
-                     const V& value) {
-    input.insert(std::make_pair(key, value));
+  template <typename IK, typename IV>
+  static bool Insert(std::unordered_map<K, V>& input, IK&& key, IV&& value) {
+    input.insert(
+        std::make_pair(std::forward<IK>(key), std::forward<IV>(value)));
     return true;
   }
 
   static void SetToEmpty(std::unordered_map<K, V>* output) { output->clear(); }
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_MAP_TRAITS_STL_H_