[mojo] Allow value deserialization to fail

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

 {%- import "interface_macros.tmpl" as interface_macros %}
 {%- import "struct_macros.tmpl" as struct_macros %}
 {%- from "enum_macros.tmpl" import is_valid_enum_def %}
 
 {%- set class_name = interface.name %}
 {%- set proxy_name = interface.name ~ "Proxy" %}
 {%- set namespace_as_string = "%s"|format(namespace|replace(".","::")) %}
 
 {%- macro alloc_params(struct, serialization_context) %}
 {%-   for param in struct.packed.packed_fields_in_ordinal_order %}
@@ skipping 206 matching lines @@
   switch (message->header()->name) {
 {%-   for method in interface.methods %}
     case internal::k{{class_name}}_{{method.name}}_Name: {
       mojo::internal::ScopedTaskTracking task_id("mojo::{{namespace_as_string}}::{{class_name}}::{{method.name}}", __FILE__, __LINE__);
 {%-     if method.response_parameters != None %}
       internal::{{class_name}}_{{method.name}}_Params_Data* params =
           reinterpret_cast<internal::{{class_name}}_{{method.name}}_Params_Data*>(
               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}}");
       sink_->{{method.name}}(
 {%- if method.parameters -%}{{pass_params(method.parameters)}}, {% endif -%}callback);
       return true;
 {%-     else %}
       break;
 {%-     endif %}
     }
@@ skipping 75 matching lines @@
     default:
       break;
   }
 
   // Unrecognized message.
   ReportValidationError(
       mojo::internal::VALIDATION_ERROR_MESSAGE_HEADER_UNKNOWN_METHOD);
   return false;
 }
 {%- endif -%}