Stop calling WritePrivateDataToCacheFileTask() on the UI thread during shutdown

chrome/browser/autocomplete/in_memory_url_index.h

Index: chrome/browser/autocomplete/in_memory_url_index.h
diff --git a/chrome/browser/autocomplete/in_memory_url_index.h b/chrome/browser/autocomplete/in_memory_url_index.h
index d5818d8c918432774574be1ca0db420a269ed4af..2b3199cf1f332db2cb2b5fd60b2818959c0ddf6a 100644
--- a/chrome/browser/autocomplete/in_memory_url_index.h
+++ b/chrome/browser/autocomplete/in_memory_url_index.h
@@ -27,6 +27,7 @@
 class HistoryQuickProviderTest;
 
 namespace base {
+class SequencedTaskRunner;
 class Time;
 }
 
@@ -287,6 +288,10 @@ class InMemoryURLIndex : public KeyedService,
   RestoreCacheObserver* restore_cache_observer_;
   SaveCacheObserver* save_cache_observer_;
 
+  // Task runner from the worker pool, used for operations which require disk
+  // access.
+  scoped_refptr<base::SequencedTaskRunner> task_runner_;
+
   base::CancelableTaskTracker private_data_tracker_;
   base::CancelableTaskTracker cache_reader_tracker_;