[mojo] Add Mojo bindings support for IPC::ParamTraits

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

Index: mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl
index 9889f8b8bcd0cd274306538a0d8fa5a3365c27e5..960518aa5dd41a56e32762799275ae5305f1b659 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl
@@ -8,7 +8,7 @@
 
 {%- macro alloc_params(struct, serialization_context) %}
 {%-   for param in struct.packed.packed_fields_in_ordinal_order %}
-  {{param.field.kind|cpp_result_type}} p_{{param.field.name}}{};
+  {{param.field.kind|cpp_wrapper_type}} p_{{param.field.name}}{};
 {%-   endfor %}
   {{struct_macros.deserialize(struct, "params", "p_%s", serialization_context)}}
 {%- endmacro %}
@@ -224,11 +224,11 @@ bool {{class_name}}Stub::AcceptWithResponder(
               message->mutable_payload());
 
       params->DecodePointersAndHandles(message->mutable_handles());
+      {{alloc_params(method.param_struct, "&serialization_context_")|indent(4)}}
       {{class_name}}::{{method.name}}Callback::Runnable* runnable =
           new {{class_name}}_{{method.name}}_ProxyToResponder(
               message->request_id(), responder);
       {{class_name}}::{{method.name}}Callback callback(runnable);
-      {{alloc_params(method.param_struct, "&serialization_context_")|indent(4)}}
       // A null |sink_| means no implementation was bound.
       assert(sink_);
       TRACE_EVENT0("mojom", "{{class_name}}::{{method.name}}");