Add support for GCM subtypes to desktop Instance ID implementation

components/gcm_driver/pref_names.cc

Index: components/gcm_driver/pref_names.cc
diff --git a/components/gcm_driver/pref_names.cc b/components/gcm_driver/pref_names.cc
new file mode 100644
index 0000000000000000000000000000000000000000..5e6a0300aa3f6b20d8aac2216f8d66893681a96b
--- /dev/null
+++ b/components/gcm_driver/pref_names.cc
@@ -0,0 +1,15 @@
+// Copyright 2016 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.
+
+#include "components/gcm_driver/pref_names.h"
+
+namespace gcm {
+namespace prefs {
+
+// A string like "com.chrome.stable.macosx" that should be used as the GCM
+// category when an app_id is sent as a subtype instead of as a category.
+const char kGCMCategoryForSubtypes[] = "gcm.category_for_subtypes";
+
+}  // namespace prefs
+}  // namespace gcm