Move Reading List preference to incognito whitelist.

Reading List preference is moved to the persistent storage list for
incognito mode.
This CL does NOT make any change of behavior.

Bug: 861722
Change-Id: Ie29e7c86cedc19842f64939a1acf60e7835bffc1
Reviewed-on: https://chromium-review.googlesource.com/1172132Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585465}

chrome/browser/prefs/pref_service_incognito_whitelist.cc

@@ -131,6 +131,9 @@ const char* const kPersistentPrefNames[] = {
     // they need to be persisted.
     rappor::prefs::kRapporCohortSeed, rappor::prefs::kRapporSecret,
 
+    // Reading list preferences are common between incognito and regular mode.
+    reading_list::prefs::kReadingListHasUnseenEntries,
+
     // Although UKMs are not collected in incognito, theses preferences may be
     // changed by UMA/Sync/Unity consent, and need to be the same between
     // incognito and regular modes.
@@ -590,9 +593,6 @@ const char* const kTemporaryIncognitoWhitelist[] = {
     onc::prefs::kDeviceOpenNetworkConfiguration,
     onc::prefs::kOpenNetworkConfiguration,
 
-    // components/reading_list/core/reading_list_pref_names.h
-    reading_list::prefs::kReadingListHasUnseenEntries,
-
     // components/search_engines/search_engines_pref_names.h
     prefs::kSyncedDefaultSearchProviderGUID,
     prefs::kDefaultSearchProviderEnabled, prefs::kSearchProviderOverrides,