Add support for GCM subtypes to desktop Instance ID implementation

components/gcm_driver/pref_names.h

Index: components/gcm_driver/pref_names.h
diff --git a/components/gcm_driver/pref_names.h b/components/gcm_driver/pref_names.h
new file mode 100644
index 0000000000000000000000000000000000000000..d17ea02326106f5336c896861107f975bca84d5c
--- /dev/null
+++ b/components/gcm_driver/pref_names.h
@@ -0,0 +1,18 @@
+// 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.
+
+#ifndef COMPONENTS_GCM_DRIVER_PREF_NAMES_H_
+#define COMPONENTS_GCM_DRIVER_PREF_NAMES_H_
+
+namespace gcm {
+namespace prefs {
+
+// Alphabetical list of preference names specific to the gcm_driver component.
+// Keep alphabetized, and document each in the .cc file.
+extern const char kGCMCategoryForSubtypes[];
+
+}  // namespace prefs
+}  // namespace gcm
+
+#endif  // COMPONENTS_GCM_DRIVER_PREF_NAMES_H_