Add GN template for generated extensions API targets.

apps/common/api/BUILD.gn

Index: apps/common/api/BUILD.gn
diff --git a/apps/common/api/BUILD.gn b/apps/common/api/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..25bd5629fb782dd8f915d80e126a181b2a1b13eb
--- /dev/null
+++ b/apps/common/api/BUILD.gn
@@ -0,0 +1,13 @@
+# 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.
+
+import("//extensions/generated_extensions_api.gni")
+
+generated_extensions_api("apps_api") {
+  schemas = [
+    "app_runtime.idl",
+  ]
+  root_namespace = "extensions::apps_api"
+  impl_dir = "//apps/browser/api"
+}