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 d1637395e5b193f3ec6d6498a29067d39f49e5d3..e2bf68a7361eae974db5d25792de8b9862248ea4 100644
--- a/mojo/public/tools/bindings/mojom.gni
+++ b/mojo/public/tools/bindings/mojom.gni
@@ -13,6 +13,8 @@
 #
 #   deps (optional)
 #
+#   testonly (optional)
+#
 #   visibility (optional)
 #       Normal meaning. However, this must be a list (normal visibility
 #       allows a single string).
@@ -109,6 +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 = [