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

mojo/public/cpp/bindings/map_traits.h

Index: mojo/public/cpp/bindings/map_traits.h
diff --git a/mojo/public/cpp/bindings/map_traits.h b/mojo/public/cpp/bindings/map_traits.h
index 7aa1cbf5507cfe5a521c2e922421508ae65ada4a..01dd66d6a69a20fbcaaaf1c748e81eb4f579dd50 100644
--- a/mojo/public/cpp/bindings/map_traits.h
+++ b/mojo/public/cpp/bindings/map_traits.h
@@ -36,10 +36,12 @@ namespace mojo {
 //     static V& GetValue(CustomIterator& iterator);
 //     static const V& GetValue(CustomConstIterator& iterator);
 //
-//     static void Insert(CustomMap<K, V>& input,
+//     // Returning false results in deserialization failure and causes the
+//     // message pipe receiving it to be disconnected.
+//     static bool Insert(CustomMap<K, V>& input,
 //                        const K& key,
 //                        V&& value);
-//     static void Insert(CustomMap<K, V>& input,
+//     static bool Insert(CustomMap<K, V>& input,
 //                        const K& key,
 //                        const V& value);
 //