Add GN template for generated extensions API targets.

tools/json_schema_compiler/BUILD.gn

Index: tools/json_schema_compiler/BUILD.gn
diff --git a/components/language_usage_metrics/BUILD.gn b/tools/json_schema_compiler/BUILD.gn
similarity index 53%
copy from components/language_usage_metrics/BUILD.gn
copy to tools/json_schema_compiler/BUILD.gn
index 88ad430bd5d7fa51b30734c134e38914e8ab72cd..2d7c183807d7ca129b8bf71b94ffac9ef123eb74 100644
--- a/components/language_usage_metrics/BUILD.gn
+++ b/tools/json_schema_compiler/BUILD.gn
@@ -2,13 +2,12 @@
 # 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",
-  ]
-
-  deps = [
-    "//base",
+    "util.cc",
+    "util.h"
   ]
+  deps = [ "//base" ]
 }
+