Do not observe MessageLoop destruction from ModelSafeWorker.

components/sync/driver/glue/sync_backend_host_impl.cc

 // Copyright 2013 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.
 
 #include "components/sync/driver/glue/sync_backend_host_impl.h"
 
 #include <utility>
 
+#include "base/bind.h"
 #include "base/command_line.h"
 #include "base/feature_list.h"
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "components/invalidation/public/invalidation_service.h"
 #include "components/invalidation/public/object_id_invalidation_map.h"
 #include "components/signin/core/browser/signin_client.h"
 #include "components/sync/base/experiments.h"
@@ skipping 215 matching lines @@
   core_->ShutdownOnUIThread();
 }
 
 std::unique_ptr<base::Thread> SyncBackendHostImpl::Shutdown(
     ShutdownReason reason) {
   // StopSyncingForShutdown() (which nulls out |frontend_|) should be
   // called first.
   DCHECK(!frontend_);
   DCHECK(registrar_->sync_thread()->IsRunning());
 
-  bool sync_thread_claimed = (reason != BROWSER_SHUTDOWN);
-
   if (invalidation_handler_registered_) {
     if (reason == DISABLE_SYNC) {
       UnregisterInvalidationIds();
     }
     invalidator_->UnregisterInvalidationHandler(this);
     invalidator_ = nullptr;
   }
   invalidation_handler_registered_ = false;
 
   model_type_connector_.reset();
 
-  // Shut down and destroy sync manager.
+  // Shut down and destroy SyncManager. SyncManager holds a pointer to
+  // |registrar_| so its destruction must be sequenced before the destruction of
+  // |registrar_|.
   registrar_->sync_thread()->task_runner()->PostTask(
       FROM_HERE,
       base::Bind(&SyncBackendHostCore::DoShutdown, core_, reason));
   core_ = nullptr;
 
-  // Worker cleanup.
-  SyncBackendRegistrar* detached_registrar = registrar_.release();
-  detached_registrar->sync_thread()->task_runner()->PostTask(
-      FROM_HERE, base::Bind(&SyncBackendRegistrar::Shutdown,
-                            base::Unretained(detached_registrar)));
+  // Destroy |registrar_|.
+  std::unique_ptr<base::Thread> released_sync_thread =
+      registrar_->ReleaseSyncThread();
+  released_sync_thread->task_runner()->DeleteSoon(FROM_HERE,
+                                                  registrar_.release());
 
-  if (sync_thread_claimed)
-    return detached_registrar->ReleaseSyncThread();
-  else
-    return std::unique_ptr<base::Thread>();
+  return released_sync_thread;
 }
 
 void SyncBackendHostImpl::UnregisterInvalidationIds() {
   if (invalidation_handler_registered_) {
     CHECK(invalidator_->UpdateRegisteredInvalidationIds(this, ObjectIdSet()));
   }
 }
 
 ModelTypeSet SyncBackendHostImpl::ConfigureDataTypes(
     ConfigureReason reason,
@@ skipping 534 matching lines @@
     const SyncManager::ClearServerDataCallback& frontend_callback) {
   DCHECK(ui_thread_->BelongsToCurrentThread());
   frontend_callback.Run();
 }
 
 }  // namespace syncer
 
 #undef SDVLOG
 
 #undef SLOG