Destroy store on task runner. That's where all store operations happen.

components/safe_browsing_db/v4_database.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 <memory>
 
 #include "base/callback.h"
 #include "base/debug/leak_annotations.h"
 #include "base/files/file_util.h"
 #include "base/memory/ptr_util.h"
@@ skipping 124 matching lines @@
     current_task_runner->PostTask(FROM_HERE, db_updated_callback_);
     db_updated_callback_.Reset();
   }
 }
 
 void V4Database::UpdatedStoreReady(ListIdentifier identifier,
                                    std::unique_ptr<V4Store> new_store) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   DCHECK(pending_store_updates_);
   if (new_store) {
+    V4Store::Destroy(std::move((*store_map_)[identifier]));
     (*store_map_)[identifier] = std::move(new_store);

[Scott Hess - ex-Googler, 2016/10/18 02:57:01]

Does this work?

I mean, I think it probably does, but I'm having troubles describing how.  I'll
just throw this out there, but leave it in your hands whether you want to go
there:

std::swap((*store_map_)[identifier], new_store);
if (new_store) {
  V4Store::Destroy(std::move(new_store));
}

Of course, abusing new_store as temp storage like this is optional, and I
haven't read Destroy() yet to see if that's where the null handling should be.

[Scott Hess - ex-Googler, 2016/10/18 02:57:55]

Or, of course:

(*store_map_)[identifier].swap(new_store);

   }
 
   pending_store_updates_--;
   if (!pending_store_updates_) {
     db_updated_callback_.Run();
     db_updated_callback_.Reset();
   }
 }
 
 std::unique_ptr<StoreStateMap> V4Database::GetStoreStateMap() {
@@ skipping 64 matching lines @@
       filename_(filename),
       list_id_(list_id),
       sb_threat_type_(sb_threat_type) {
   DCHECK(!fetch_updates_ || !filename_.empty());
   DCHECK_NE(SB_THREAT_TYPE_SAFE, sb_threat_type_);
 }
 
 ListInfo::~ListInfo() {}
 
 }  // namespace safe_browsing