Enable use_once_callback on mojom GN rule by default

mojo/public/tools/bindings/mojom.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 declare_args() {
   # Indicates whether typemapping should be supported in this build
   # configuration. This may be disabled when building external projects which
   # depend on //mojo but which do not need/want all of the Chromium tree
   # dependencies that come with typemapping.
   #
@@ skipping 108 matching lines @@
 #
 #   visibility (optional)
 #
 #   visibility_blink (optional)
 #       The value to use for visibility for the blink variant. If unset,
 #       |visibility| is used.
 #
 #   use_once_callback (optional)
 #       If set to true, generated classes will use base::OnceCallback instead of
 #       base::RepeatingCallback.
-#       Default value is false.
+#       Default value is true.
 #       TODO(dcheng):
 #           - Convert everything to use OnceCallback.
 #           - Remove support for the old mode.
 #
 #   cpp_only (optional)
 #       If set to true, only the C++ bindings targets will be generated.
 #
 #   use_new_js_bindings (optional)
 #       If set to true, the generated JS code will use the new module loading
 #       approach and the core API exposed by Web IDL.
@@ skipping 291 matching lines @@
           if (defined(invoker.export_class_attribute)) {
             args += [
               "--export_attribute",
               invoker.export_class_attribute,
               "--export_header",
               invoker.export_header,
             ]
           }
         }
 
-        if (defined(invoker.use_once_callback) && invoker.use_once_callback) {
+        if (!defined(invoker.use_once_callback) || invoker.use_once_callback) {
           args += [ "--use_once_callback" ]
         }
 
         if (defined(invoker.use_new_js_bindings) &&
             invoker.use_new_js_bindings) {
           args += [ "--use_new_js_bindings" ]
         }
       }
     }
 
@@ skipping 203 matching lines @@
           full_name = get_label_info(d, "label_no_toolchain")
           deps += [ "${full_name}_java" ]
         }
 
         srcjar_deps = [ ":$java_srcjar_target_name" ]
         run_findbugs_override = false
       }
     }
   }
 }