Add GN template for generated extensions API targets.

tools/json_schema_compiler/BUILD.gn

 # 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.
 
-source_set("language_usage_metrics") {
+# Utility sources against which generated API modules should be linked.
+source_set("generated_api_util") {
   sources = [
-    "language_usage_metrics.cc",
-    "language_usage_metrics.h",
+    "util.cc",
+    "util.h"
   ]
+  deps = [ "//base" ]
+}
 
-  deps = [
-    "//base",
-  ]
-}