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

mojo/public/cpp/bindings/map_traits_stl.h

Index: mojo/public/cpp/bindings/map_traits_stl.h
diff --git a/mojo/public/cpp/bindings/map_traits_stl.h b/mojo/public/cpp/bindings/map_traits_stl.h
index 25ae65a8384108e7277bce3e3dfce81543ed8c18..2d5c137ff355d1526a7561d6b64bb6d010309865 100644
--- a/mojo/public/cpp/bindings/map_traits_stl.h
+++ b/mojo/public/cpp/bindings/map_traits_stl.h
@@ -44,11 +44,13 @@ struct MapTraits<std::map<K, V>> {
   static V& GetValue(Iterator& iterator) { return iterator->second; }
   static const V& GetValue(ConstIterator& iterator) { return iterator->second; }
 
-  static void Insert(std::map<K, V>& input, const K& key, V&& value) {
+  static bool Insert(std::map<K, V>& input, const K& key, V&& value) {
     input.insert(std::make_pair(key, std::forward<V>(value)));
+    return true;
   }
-  static void Insert(std::map<K, V>& input, const K& key, const V& value) {
+  static bool Insert(std::map<K, V>& input, const K& key, const V& value) {
     input.insert(std::make_pair(key, value));
+    return true;
   }
 
   static void SetToEmpty(std::map<K, V>* output) { output->clear(); }