[NaCl Docs] Add some messaging system docs for how to use pp::VarDictionaries.

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

 .. _messaging_system:
 
 ################
 Messaging System
 ################
 
 .. contents::
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   :backlinks: none
   :depth: 2
@@ skipping 340 matching lines @@
 
 The Native Client module sends messages back to the JavaScript code
 using ``PostMessage()``. The Native Client module always returns
 its values in the form of a ``pp::Var`` that can be processed by the
 browser's JavaScript. In this example, the message is posted at the
 end of the Native Client module's ``HandleMessage()`` function:
 
 .. naclcode::
 
   PostMessage(var_reply);
+
+
+Sending and receiving other ``pp::Var`` types
+---------------------------------------------
+
+Besides strings, ``pp::Var`` can represent other types of JavaScript
+objects. For example, messages can be JavaScript dictionaries.

[binji, 2013/09/24 21:56:26]

JavaScript doesn't really have dictionaries, just objects.

[jvoung (off chromium), 2013/09/24 22:35:53]

Done.

+These richer types can make it easier to implement an application's
+messaging protocol.
+
+To send a dictionary from the NaCl module to JavaScript simply
+create a ``pp::VarDictionary`` and then call ``PostMessage`` with
+the dictionary.
+
+.. naclcode::
+
+  pp::VarDictionary dictionary;
+  dictionary.Set(pp::Var("command"), pp::Var(next_command));
+  dictionary.Set(pp::Var("param_int"), pp::Var(123));
+  pp::VarArray an_array;
+  an_array.Set(0, pp::Var("string0"));
+  an_array.Set(1, pp::Var("string1"))
+  dictionary.Set(pp::Var("param_array"), an_array);
+  PostMessage(dictionary);

[binji, 2013/09/24 21:56:26]

Is it worth mentioning what this will look like in JavaScript?

{
  command: next_command,
  param_int: 123,
  param_array: ['string0', 'string1']
}

[jvoung (off chromium), 2013/09/24 22:35:53]

Seems worth doing -- Done.

+
+
+To receive a dictionary-typed message in the NaCl module, test that
+the message is truly a dictionary type, then convert the message
+with the ``pp::VarDictionary`` class.
+
+.. naclcode::
+
+  virtual void HandleMessage(const pp::Var& var) {
+    if (var.is_dictionary()) {
+      pp::VarDictionary dictionary(var);
+      // Use the dictionary
+      pp::VarArray keys = dictionary.GetKeys();
+      // ...
+    } else {
+      // ...
+    }
+  }