Mojo C++ bindings: add a "deprecated" comment to type_converter.h

mojo/public/cpp/bindings/type_converter.h

Index: mojo/public/cpp/bindings/type_converter.h
diff --git a/mojo/public/cpp/bindings/type_converter.h b/mojo/public/cpp/bindings/type_converter.h
index e17ac861c3ec1c919764e60611b797fec066c2a6..395eeb4ffeeb4d2bf89564a689b2f97d222293de 100644
--- a/mojo/public/cpp/bindings/type_converter.h
+++ b/mojo/public/cpp/bindings/type_converter.h
@@ -11,6 +11,11 @@
 
 namespace mojo {
 
+// NOTE: TypeConverter is deprecated. Please consider StructTraits /
+// UnionTraits / EnumTraits / ArrayTraits / MapTraits / StringTraits if you
+// would like to convert between custom types and the wire format of mojom
+// types.
+//
 // Specialize the following class:
 //   template <typename T, typename U> struct TypeConverter;
 // to perform type conversion for Mojom-defined structs and arrays. Here, T is