[Sync] Make SESSIONS an implicit type

chrome/browser/sync/sync_prefs.cc

Index: chrome/browser/sync/sync_prefs.cc
diff --git a/chrome/browser/sync/sync_prefs.cc b/chrome/browser/sync/sync_prefs.cc
index 42518ef17cc7ea97e0b29ef3cb7caf74c1be3c0b..ba0ab7042c404917800bce765a514dbf0337f190 100644
--- a/chrome/browser/sync/sync_prefs.cc
+++ b/chrome/browser/sync/sync_prefs.cc
@@ -319,6 +319,8 @@ const char* SyncPrefs::GetPrefNameForDataType(syncer::ModelType data_type) {
       return prefs::kSyncHistoryDeleteDirectives;
     case syncer::SYNCED_NOTIFICATIONS:
       return prefs::kSyncSyncedNotifications;
+    case syncer::TABS:
+      return prefs::kSyncTabs;
     default:
       break;
   }
@@ -385,8 +387,9 @@ void SyncPrefs::RegisterPrefGroups() {
 
   pref_groups_[syncer::PREFERENCES].Put(syncer::SEARCH_ENGINES);
 
-  // TODO(akalin): Revisit this once UI lands.
-  pref_groups_[syncer::SESSIONS].Put(syncer::HISTORY_DELETE_DIRECTIVES);
+  pref_groups_[syncer::TYPED_URLS].Put(syncer::HISTORY_DELETE_DIRECTIVES);
+  pref_groups_[syncer::HISTORY_DELETE_DIRECTIVES].Put(syncer::SESSIONS);
+  pref_groups_[syncer::TABS].Put(syncer::SESSIONS);
 }
 
 // static
@@ -412,6 +415,13 @@ bool SyncPrefs::GetDataTypePreferred(syncer::ModelType type) const {
     NOTREACHED();
     return false;
   }
+  if (type == syncer::TABS &&
+      pref_service_->GetUserPrefValue(pref_name) == NULL &&
+      pref_service_->IsUserModifiablePreference(pref_name)) {
+    // If there is no tab sync preference yet (i.e. newly enabled type),
+    // default to the session sync preference value.
+    pref_name = GetPrefNameForDataType(syncer::SESSIONS);
+  }
 
   return pref_service_->GetBoolean(pref_name);
 }
@@ -437,8 +447,6 @@ syncer::ModelTypeSet SyncPrefs::ResolvePrefGroups(
       i != pref_groups_.end(); ++i) {
     if (types.Has(i->first))
       types_with_groups.PutAll(i->second);
-    else
-      types_with_groups.RemoveAll(i->second);
   }
   types_with_groups.RetainAll(registered_types);
   return types_with_groups;