[Sync] Move sync thread ownership to ProfileSyncService.

components/sync/driver/glue/sync_backend_registrar.cc

Index: components/sync/driver/glue/sync_backend_registrar.cc
diff --git a/components/sync/driver/glue/sync_backend_registrar.cc b/components/sync/driver/glue/sync_backend_registrar.cc
index 3c832bbea7838e4fc763b147000db3f340944860..6d903a59f1a7f28c460b11086ebb9fa2af63d3bf 100644
--- a/components/sync/driver/glue/sync_backend_registrar.cc
+++ b/components/sync/driver/glue/sync_backend_registrar.cc
@@ -19,7 +19,6 @@ namespace syncer {
 SyncBackendRegistrar::SyncBackendRegistrar(
     const std::string& name,
     SyncClient* sync_client,
-    std::unique_ptr<base::Thread> sync_thread,
     const scoped_refptr<base::SingleThreadTaskRunner>& ui_thread,
     const scoped_refptr<base::SingleThreadTaskRunner>& db_thread,
     const scoped_refptr<base::SingleThreadTaskRunner>& file_thread)
@@ -31,14 +30,6 @@ SyncBackendRegistrar::SyncBackendRegistrar(
   DCHECK(ui_thread_->BelongsToCurrentThread());
   DCHECK(sync_client_);
 
-  sync_thread_ = std::move(sync_thread);
-  if (!sync_thread_) {
-    sync_thread_ = base::MakeUnique<base::Thread>("Chrome_SyncThread");
-    base::Thread::Options options;
-    options.timer_slack = base::TIMER_SLACK_MAXIMUM;
-    CHECK(sync_thread_->StartWithOptions(options));
-  }
-
   MaybeAddWorker(GROUP_DB);
   MaybeAddWorker(GROUP_FILE);
   MaybeAddWorker(GROUP_UI);
@@ -324,14 +315,6 @@ void SyncBackendRegistrar::MaybeAddWorker(ModelSafeGroup group) {
   }
 }
 
-std::unique_ptr<base::Thread> SyncBackendRegistrar::ReleaseSyncThread() {
-  return std::move(sync_thread_);
-}
-
-base::Thread* SyncBackendRegistrar::sync_thread() {
-  return sync_thread_.get();
-}
-
 ModelSafeGroup SyncBackendRegistrar::GetInitialGroupForType(
     ModelType type) const {
   return non_blocking_types_.Has(type) ? GROUP_NON_BLOCKING : GROUP_PASSIVE;