[NTP::Push] Add Content Suggestions GCM App Handler

components/ntp_snippets/pref_names.h

 // 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_NTP_SNIPPETS_PREF_NAMES_H_
 #define COMPONENTS_NTP_SNIPPETS_PREF_NAMES_H_
 
 namespace ntp_snippets {
 namespace prefs {
 
@@ skipping 72 matching lines @@
 // The pref name for the last time content suggestions were shown to the user.
 extern const char kUserClassifierLastTimeToShowSuggestions[];
 // The pref name for the last time content suggestions were used by the user.
 extern const char kUserClassifierLastTimeToUseSuggestions[];
 
 // The pref name for the current order of categories and their clicks.
 extern const char kClickBasedCategoryRankerOrderWithClicks[];
 // The pref name for the time when last click decay has happened.
 extern const char kClickBasedCategoryRankerLastDecayTime[];
 
-// The pref name for the subscription token used when subscription for breaking
-// news push updates,
+// kContentSuggestionsSubscriptionData* hold the data used when subscribing for
+// content suggestions via GCM push updates. They are stored in pref such that
+// in case of change (e.g. the token renders invalid), re-subscription is
+// required.
+///////////////////////////////////////////////////////////////////////////////
+// The pref name for the subscription token used when subscription for
+// breaking news push updates.
 extern const char kContentSuggestionsSubscriptionDataToken[];
+//////////////////////// End of kContentSuggestionsSubscriptionData *
+
+// The pref name for the subscription token received from the gcm server. As
+// recommended by the GCM team, it is cached in pref for faster bookkeeping to
+// see if subscription exists. This is pref holds the valid token even if
+// different from the one used for subscription. When they are different, Chrome
+// unsubscribes the old token from the content suggestions server, subscribe
+// with the new one and update kContentSuggestionsSubscriptionDataToken.
+extern const char kContentSuggestionsGCMSubscriptionTokenCache[];
 
 }  // namespace prefs
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_PREF_NAMES_H_