Add a wrapper for grit_action.gypi for chrome specific variables.

chrome/chrome_grit_action.gypi

+# 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.
+
+{
+  'variables': {
+    # Wrapper around build/grit_action.gypi that applies Chrome-specific grit
+    # settings. Actions that include this gypi file at their local scopes can
+    # define variables with the same names as the ones below and they will be
+    # merged.
+    'grit_additional_defines': [ '<@(chrome_grit_defines)' ],
+  },
+  'includes': [ '../build/grit_action.gypi', 'chrome_features.gypi' ],
+}