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

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.
 
 # Generate C++ and JavaScript source files from mojom files. The output files
 # will go under the generated file directory tree with the same path as each
 # input file.
 #
 # Parameters:
 #
 #   sources (required)
 #       List of source .mojom files to compile.
 #
 #   deps (optional)
 #
+#   testonly (optional)
+#
 #   visibility (optional)
 #       Normal meaning. However, this must be a list (normal visibility
 #       allows a single string).
 template("mojom") {
   assert(defined(invoker.sources),
          "\"sources\" must be defined for the $target_name template.")
 
   generator_root = "//mojo/public/tools/bindings"
   generator_script = "$generator_root/mojom_bindings_generator.py"
   generator_sources = [
@@ skipping 76 matching lines @@
       "-d", rebase_path("//", root_build_dir),
       "-I", rebase_path("//", root_build_dir),
       "-o", "{{source_gen_dir}}",
     ]
   }
 
   source_set(target_name) {
     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 = [
       ":$generator_target_name",
       "//mojo/public/cpp/bindings",
     ]
     if (defined(invoker.deps)) {
       deps += invoker.deps
     }
   }
 }