Try to flush DOMStorage too when ending the browsing session.

content/browser/storage_partition_impl.h

 // Copyright (c) 2012 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.
 
 #ifndef CONTENT_BROWSER_STORAGE_PARTITION_IMPL_H_
 #define CONTENT_BROWSER_STORAGE_PARTITION_IMPL_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 99 matching lines @@
                  const base::Time begin,
                  const base::Time end,
                  const base::Closure& callback) override;
 
   void ClearHttpAndMediaCaches(
       const base::Time begin,
       const base::Time end,
       const base::Callback<bool(const GURL&)>& url_matcher,
       const base::Closure& callback) override;
 
-  void Flush() override;
+  std::vector<scoped_refptr<base::SequencedTaskRunner>> Flush() override;
 
   // Can return nullptr while |this| is being destroyed.
   BrowserContext* browser_context() const;
 
   // Called by each renderer process once.
   void Bind(mojo::InterfaceRequest<mojom::StoragePartitionService> request);
 
   struct DataDeletionHelper;
   struct QuotaManagedDataDeletionHelper;
 
@@ skipping 111 matching lines @@
   BrowserContext* browser_context_;
 
   base::WeakPtrFactory<StoragePartitionImpl> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(StoragePartitionImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_STORAGE_PARTITION_IMPL_H_