Clean up app_shell GN build.

extensions/shell/app_shell.gni

Index: extensions/shell/app_shell.gni
diff --git a/extensions/shell/app_shell.gni b/extensions/shell/app_shell.gni
new file mode 100644
index 0000000000000000000000000000000000000000..3b5e9842b38ee6c99856b504185559bf1c41ddc9
--- /dev/null
+++ b/extensions/shell/app_shell.gni
@@ -0,0 +1,16 @@
+# Copyright 2015 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.
+
+# This file defines the app_shell gypi values. This file is read once and
+# cached, which is a performance optimization that allows us to share the
+# results of parsing the .gypi file between all app_shell BUILD.gn files.
+# It also saves us from duplicating this exec_script call.
+app_shell_gypi_values =
+    exec_script("//build/gypi_to_gn.py",
+                [
+                  rebase_path("app_shell.gypi"),
+                  "--replace=<(SHARED_INTERMEDIATE_DIR)=$root_gen_dir",
+                ],
+                "scope",
+                [ "app_shell.gypi" ])