Add migration to WebAssembly information to PNaCl / NaCl docs.

native_client_sdk/src/doc/devguide/coding/message-system.rst

 .. _message-system:
 
+.. include:: /migration/deprecation.inc
+
 ################
 Messaging System
 ################
 
 .. contents::
   :local:
   :backlinks: none
   :depth: 2
 
 This section describes the messaging system used to communicate between the
@@ skipping 391 matching lines @@
   virtual void HandleMessage(const pp::Var& var) {
     if (var.is_dictionary()) {
       pp::VarDictionary dictionary(var);
       // Use the dictionary
       pp::VarArray keys = dictionary.GetKeys();
       // ...
     } else {
       // ...
     }
   }