GN: Fix compile errors with os==chromeos mojo/public build

mojo/public/tools/bindings/mojom.gni

Index: mojo/public/tools/bindings/mojom.gni
diff --git a/mojo/public/tools/bindings/mojom.gni b/mojo/public/tools/bindings/mojom.gni
index 74e6153a9c093d9dfeb4b32d402afff755364850..e12d8e0af06daefd69d5354e352909e65f127487 100644
--- a/mojo/public/tools/bindings/mojom.gni
+++ b/mojo/public/tools/bindings/mojom.gni
@@ -12,7 +12,11 @@
 #       List of source .mojom files to compile.
 #
 #   deps (optional)
+#
 #   public_deps (optional)
+#
+#   testonly (optional)
+#
 #   visibility (optional)
 template("mojom") {
   assert(defined(invoker.sources),
@@ -107,6 +111,9 @@ template("mojom") {
     if (defined(invoker.visibility)) {
       visibility = invoker.visibility
     }
+    if (defined(invoker.testonly)) {
+      testonly = invoker.testonly
+    }
     sources = process_file_template(invoker.sources, generator_cpp_outputs)
     data = process_file_template(invoker.sources, generator_js_outputs)
     deps = [