Mojo C++ bindings: introduce mojo::WTFMap for blink bindings.

mojo/public/cpp/bindings/map_traits_standard.h

Index: mojo/public/cpp/bindings/map_traits_standard.h
diff --git a/mojo/public/cpp/bindings/map_traits_standard.h b/mojo/public/cpp/bindings/map_traits_standard.h
index 231c739a79de218401a68805c915137cb48560a6..0c768903ec96d0937e90dd2277480c79aef7266f 100644
--- a/mojo/public/cpp/bindings/map_traits_standard.h
+++ b/mojo/public/cpp/bindings/map_traits_standard.h
@@ -36,11 +36,13 @@ struct MapTraits<Map<K, V>> {
   static V& GetValue(Iterator& iterator) { return iterator->second; }
   static const V& GetValue(ConstIterator& iterator) { return iterator->second; }
 
-  static void Insert(Map<K, V>& input, const K& key, V&& value) {
+  static bool Insert(Map<K, V>& input, const K& key, V&& value) {
     input.insert(key, std::forward<V>(value));
+    return true;
   }
-  static void Insert(Map<K, V>& input, const K& key, const V& value) {
+  static bool Insert(Map<K, V>& input, const K& key, const V& value) {
     input.insert(key, value);
+    return true;
   }
 
   static void SetToEmpty(Map<K, V>* output) { output->SetToEmpty(); }