ml: add forward_declaration to mojom/type_mappings.json

mojom_cpp_generator.py will expect this field when libchrome uprev to
after r899547, default value is None (null in json).

BUG=b:198993705
TEST=emerge-hatch ml

Change-Id: I93e1d16a88a36d961ee9755b08c6dbcdcfa29dab
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/3272252
Tested-by: Grace Cham <hscham@chromium.org>
Reviewed-by: Honglin Yu <honglinyu@chromium.org>
Commit-Queue: Grace Cham <hscham@chromium.org>
diff --git a/ml/mojom/type_mappings.json b/ml/mojom/type_mappings.json
index fd2dab1..a39bf39 100644
--- a/ml/mojom/type_mappings.json
+++ b/ml/mojom/type_mappings.json
@@ -10,6 +10,7 @@
     "mojo_base.mojom.Time": {
       "hashable": false,
       "typename": "base::Time",
+      "forward_declaration": null,
       "traits_headers": [
         "ml/mojom/time_mojom_traits.h"
       ],
@@ -25,6 +26,7 @@
     "mojo_base.mojom.TimeDelta": {
       "hashable": false,
       "typename": "base::TimeDelta",
+      "forward_declaration": null,
       "traits_headers": [
         "ml/mojom/time_mojom_traits.h"
       ],