Store a version number in the localstorage database.

content/browser/leveldb_wrapper_impl.cc

 // Copyright 2016 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 "content/browser/leveldb_wrapper_impl.h"
 
 #include "base/bind.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "components/leveldb/public/cpp/util.h"
 #include "content/public/browser/browser_thread.h"
@@ skipping 225 matching lines @@
     return;
   no_bindings_callback_.Run();
 }
 
 void LevelDBWrapperImpl::LoadMap(const base::Closure& completion_callback) {
   DCHECK(!map_);
   on_load_complete_tasks_.push_back(completion_callback);
   if (on_load_complete_tasks_.size() > 1)
     return;
 
+  if (!database_) {
+    OnLoadComplete(leveldb::mojom::DatabaseError::IO_ERROR,
+                   std::vector<leveldb::mojom::KeyValuePtr>());
+    return;
+  }
+
   // TODO(michaeln): Import from sqlite localstorage db.
   database_->GetPrefixed(prefix_,
                          base::Bind(&LevelDBWrapperImpl::OnLoadComplete,
                                     weak_ptr_factory_.GetWeakPtr()));
 }
 
 void LevelDBWrapperImpl::OnLoadComplete(
     leveldb::mojom::DatabaseError status,
     std::vector<leveldb::mojom::KeyValuePtr> data) {
   DCHECK(!map_);
@@ skipping 111 matching lines @@
                               weak_ptr_factory_.GetWeakPtr()));
 }
 
 void LevelDBWrapperImpl::OnCommitComplete(leveldb::mojom::DatabaseError error) {
   // TODO(michaeln): What if it fails, uma here or in the DB class?
   --commit_batches_in_flight_;
   StartCommitTimer();
 }
 
 }  // namespace content