mojom: Add associative arrays to the mojom language.

mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl

 {%- import "interface_macros.tmpl" as interface_macros %}
 {%- set class_name = interface.name %}
 {%- set proxy_name = interface.name ~ "Proxy" %}
 {%- set namespace_as_string = "%s"|format(namespace|replace(".","::")) %}
 
 {%- macro alloc_params(parameters) %}
 {%-   for param in parameters %}
 {%-     if param.kind|is_object_kind  %}
 {{param.kind|cpp_result_type}} p{{loop.index}};
 Deserialize_(params->{{param.name}}.ptr, &p{{loop.index}});
@@ skipping 37 matching lines @@
   {# TODO(yzshen): Consider refactoring to share code with
      struct_serialization_definition.tmpl #}
   {{params_name}}* params =
       {{params_name}}::New(builder.buffer());
 {#--- Sets #}
 {%   for param in parameters %}
 {%-     if param.kind|is_object_kind %}
 {%-       if param.kind|is_any_array_kind %}
   mojo::SerializeArray_<{{param.kind|get_array_validate_params|indent(24)}}>(
       mojo::internal::Forward(in_{{param.name}}), builder.buffer(), &params->{{param.name}}.ptr);
+{%-       elif param.kind|is_map_kind %}
+  mojo::SerializeMap_<{{param.kind.value_kind|get_map_validate_params|indent(24)}}>(
+      mojo::internal::Forward(in_{{param.name}}), builder.buffer(), &params->{{param.name}}.ptr);
 {%-       else %}
   Serialize_(mojo::internal::Forward(in_{{param.name}}), builder.buffer(), &params->{{param.name}}.ptr);
 {%-       endif %}
 {%-       if not param.kind|is_nullable_kind %}
   MOJO_INTERNAL_DLOG_SERIALIZATION_WARNING(
       !params->{{param.name}}.ptr,
       mojo::internal::VALIDATION_ERROR_UNEXPECTED_NULL_POINTER,
       "null {{param.name}} argument in {{params_description}}");
 {%-       endif %}
 {%-     elif param.kind|is_any_handle_kind %}
@@ skipping 257 matching lines @@
       }
       break;
     }
 {%-   endfor %}
   }
 {%- endif %}
 
   return sink_->Accept(message);
 }
 {%- endif -%}