Do not observe MessageLoop destruction from ModelSafeWorker.

components/sync/driver/glue/sync_backend_host_impl_unittest.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 <cstddef>
 #include <utility>
 
 #include "base/files/file_util.h"
@@ skipping 124 matching lines @@
  private:
   ModelTypeSet initial_sync_ended_types_;
   ModelTypeSet progress_marker_types_;
   ModelTypeSet configure_fail_types_;
   FakeSyncManager** fake_manager_;
 };
 
 class BackendSyncClient : public FakeSyncClient {
  public:
   scoped_refptr<ModelSafeWorker> CreateModelWorkerForGroup(
-      ModelSafeGroup group,
-      WorkerLoopDestructionObserver* observer) override {
+      ModelSafeGroup group) override {
     switch (group) {
       case GROUP_PASSIVE:
-        return new PassiveModelWorker(observer);
+        return new PassiveModelWorker();
       default:
         return nullptr;
     }
   }
 };
 
 class SyncBackendHostTest : public testing::Test {
  protected:
   SyncBackendHostTest() : fake_manager_(nullptr) {}
 
@@ skipping 670 matching lines @@
   backend_->StopSyncingForShutdown();
   // Verify that call to DeactivateNonBlockingDataType doesn't assert.
   backend_->DeactivateNonBlockingDataType(AUTOFILL);
   backend_->Shutdown(STOP_SYNC);
   backend_.reset();
 }
 
 }  // namespace
 
 }  // namespace syncer