Revert of Document that HistoryBackend is sequence-affine, not thread-affine.

components/browser_sync/profile_sync_service_typed_url_unittest.cc

Index: components/browser_sync/profile_sync_service_typed_url_unittest.cc
diff --git a/components/browser_sync/profile_sync_service_typed_url_unittest.cc b/components/browser_sync/profile_sync_service_typed_url_unittest.cc
index 8e72b88d68310e1ae8a8ce5da160396c9070f00d..da6c619b817cc37ef0839c5b8ce35ada11bdabbb 100644
--- a/components/browser_sync/profile_sync_service_typed_url_unittest.cc
+++ b/components/browser_sync/profile_sync_service_typed_url_unittest.cc
@@ -112,8 +112,6 @@
 
   ~HistoryServiceMock() override {}
 
-  void ReleaseBackend() { backend_ = nullptr; }
-
   void set_task_runner(
       scoped_refptr<base::SingleThreadTaskRunner> task_runner) {
     DCHECK(task_runner.get());
@@ -212,11 +210,6 @@
 
   ~ProfileSyncServiceTypedUrlTest() override {
     history_service_->Shutdown();
-
-    // Release the HistoryBackend so it's deleted on data_type_thread() as part
-    // of DeleteSyncableService() instead of on the main thread when
-    // HistoryService is deleted.
-    history_service_->ReleaseBackend();
 
     // Request stop to get deletion tasks related to the HistoryService posted
     // on the Sync thread. It is important to not Shutdown at this moment,