Make prefs::kSyncDemographicsBirthYearOffset syncable.

This CL updates prefs::kSyncDemographicsBirthYearOffset to be
syncable. This change prevents a user who resets their profile
data without changing their UMA client id from generating a
new random offset, which could narrow down the true birth
year value.

Bug: 978849
Change-Id: I34d347dfdf5a35381443baa327883c72fb0aaed3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1803763
Commit-Queue: Roger McFarlane <rogerm@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Tatiana Gornak <melandory@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696895}

components/sync/base/pref_names.cc

@@ -135,9 +135,11 @@ const char kSyncDemographics_GenderPath[] = "gender";
 
 // Stores a "secret" offset that is used to randomize the birth year for metrics
 // reporting. This value should not be logged to UMA directly; instead, it
-// should be summed with the kSyncDemographicsBirthYear. This value is both
-// generated and stored locally on the client and is not known outside of the
-// client. It is not synced.
+// should be summed with the kSyncDemographicsBirthYear. This value is generated
+// locally on the client the first time a user begins to merge birth year data
+// into their UMA reports. The value is synced to the user's other devices so
+// that the user consistently uses the same offset across login/logout events
+// and after clearing their other browser data.
 const char kSyncDemographicsBirthYearOffset[] =
     "sync.demographics_birth_year_offset";
 

components/sync/base/sync_prefs.cc

@@ -127,7 +127,7 @@ const char* GetPrefNameForType(UserSelectableType type) {
 
 // Gets an offset to add noise to the birth year. If not present in prefs, the
 // offset will be randomly generated within the offset range and cached in
-// prefs.
+// syncable prefs.
 int GetBirthYearOffset(PrefService* pref_service) {
   int offset =
       pref_service->GetInteger(prefs::kSyncDemographicsBirthYearOffset);
@@ -295,7 +295,8 @@ void SyncPrefs::RegisterProfilePrefs(
       user_prefs::PrefRegistrySyncable::SYNCABLE_PRIORITY_PREF);
   registry->RegisterIntegerPref(
       prefs::kSyncDemographicsBirthYearOffset,
-      kUserDemographicsBirthYearNoiseOffsetDefaultValue);
+      kUserDemographicsBirthYearNoiseOffsetDefaultValue,
+      user_prefs::PrefRegistrySyncable::SYNCABLE_PREF);
 
   // Obsolete prefs that will be removed after a grace period.
   RegisterObsoleteUserTypePrefs(registry);