Make mojo::Callback safe to copy across threads.

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

 {%- import "interface_macros.tmpl" as interface_macros %}
 {%- import "struct_macros.tmpl" as struct_macros %}
 
 {%- 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) %}
   bool success = true;
 {%-   for param in struct.packed.packed_fields_in_ordinal_order %}
@@ skipping 182 matching lines @@
 {%-   if method.response_parameters != None %}
 {%-     set message_name =
             "internal::k%s_%s_Name"|format(interface.name, method.name) %}
 {%-     set response_params_struct = method.response_param_struct %}
 {%-     set params_description =
             "%s.%s response"|format(interface.name, method.name) %}
 class {{class_name}}_{{method.name}}_ProxyToResponder
     : public {{class_name}}::{{method.name}}Callback::Runnable {
  public:
   ~{{class_name}}_{{method.name}}_ProxyToResponder() override {
-    // Is the Mojo application destroying the callback without running it
-    // and without first closing the pipe?
-    bool callback_was_dropped = responder_ && responder_->IsValid();
+#if DCHECK_IS_ON()
+    if (responder_) {
+      // Is the Mojo application destroying the callback without running it
+      // and without first closing the pipe?
+      responder_->DCheckInvalid("The callback passed to "
+          "{{class_name}}::{{method.name}}({%- if method.parameters -%}{{pass_params(method.parameters)}}, {% endif -%}callback) "
+          "was never run.");
+    }
+#endif
     // If the Callback was dropped then deleting the responder will close
     // the pipe so the calling application knows to stop waiting for a reply.
     delete responder_;
-    DCHECK(!callback_was_dropped)  << "The callback passed to "
-        "{{class_name}}::{{method.name}}({%- if method.parameters -%}{{pass_params(method.parameters)}}, {% endif -%}callback) "
-        "was never run.";
   }
 
   {{class_name}}_{{method.name}}_ProxyToResponder(
       uint64_t request_id,
       bool is_sync,
       mojo::MessageReceiverWithStatus* responder,
       scoped_refptr<mojo::internal::MultiplexRouter> router)
       : request_id_(request_id),
         is_sync_(is_sync),
         responder_(responder),
@@ skipping 178 matching lines @@
     default:
       break;
   }
 
   // Unrecognized message.
   ReportValidationError(
       mojo::internal::VALIDATION_ERROR_MESSAGE_HEADER_UNKNOWN_METHOD);
   return false;
 }
 {%- endif -%}