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 da6c619b817cc37ef0839c5b8ce35ada11bdabbb..8e72b88d68310e1ae8a8ce5da160396c9070f00d 100644
--- a/components/browser_sync/profile_sync_service_typed_url_unittest.cc
+++ b/components/browser_sync/profile_sync_service_typed_url_unittest.cc
@@ -112,6 +112,8 @@ class HistoryServiceMock : public history::HistoryService {
 
   ~HistoryServiceMock() override {}
 
+  void ReleaseBackend() { backend_ = nullptr; }
+
   void set_task_runner(
       scoped_refptr<base::SingleThreadTaskRunner> task_runner) {
     DCHECK(task_runner.get());
@@ -211,6 +213,11 @@ class ProfileSyncServiceTypedUrlTest : public AbstractProfileSyncServiceTest {
   ~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,
     // because after shutdown the Sync thread is not returned to the sync